diff --git a/newsfragments/4261.misc.rst b/newsfragments/4261.misc.rst
new file mode 100644
index 0000000000..83c10f0f66
--- /dev/null
+++ b/newsfragments/4261.misc.rst
@@ -0,0 +1 @@
+Avoid implicit ``encoding`` parameter in ``setuptools/tests``.
diff --git a/setuptools/tests/config/test_apply_pyprojecttoml.py b/setuptools/tests/config/test_apply_pyprojecttoml.py
index 555489b140..2ca35759bc 100644
--- a/setuptools/tests/config/test_apply_pyprojecttoml.py
+++ b/setuptools/tests/config/test_apply_pyprojecttoml.py
@@ -44,8 +44,9 @@ def test_apply_pyproject_equivalent_to_setupcfg(url, monkeypatch, tmp_path):
     monkeypatch.setattr(expand, "read_attr", Mock(return_value="0.0.1"))
     setupcfg_example = retrieve_file(url)
     pyproject_example = Path(tmp_path, "pyproject.toml")
-    toml_config = Translator().translate(setupcfg_example.read_text(), "setup.cfg")
-    pyproject_example.write_text(toml_config)
+    setupcfg_text = setupcfg_example.read_text(encoding="utf-8")
+    toml_config = Translator().translate(setupcfg_text, "setup.cfg")
+    pyproject_example.write_text(toml_config, encoding="utf-8")
 
     dist_toml = pyprojecttoml.apply_configuration(makedist(tmp_path), pyproject_example)
     dist_cfg = setupcfg.apply_configuration(makedist(tmp_path), setupcfg_example)
@@ -177,9 +178,9 @@ def _pep621_example_project(
         text = text.replace(orig, subst)
     pyproject.write_text(text, encoding="utf-8")
 
-    (tmp_path / readme).write_text("hello world")
-    (tmp_path / "LICENSE.txt").write_text("--- LICENSE stub ---")
-    (tmp_path / "spam.py").write_text(PEP621_EXAMPLE_SCRIPT)
+    (tmp_path / readme).write_text("hello world", encoding="utf-8")
+    (tmp_path / "LICENSE.txt").write_text("--- LICENSE stub ---", encoding="utf-8")
+    (tmp_path / "spam.py").write_text(PEP621_EXAMPLE_SCRIPT, encoding="utf-8")
     return pyproject
 
 
diff --git a/setuptools/tests/config/test_expand.py b/setuptools/tests/config/test_expand.py
index cdcbffc14c..fe80890678 100644
--- a/setuptools/tests/config/test_expand.py
+++ b/setuptools/tests/config/test_expand.py
@@ -12,7 +12,7 @@ def write_files(files, root_dir):
     for file, content in files.items():
         path = root_dir / file
         path.parent.mkdir(exist_ok=True, parents=True)
-        path.write_text(content)
+        path.write_text(content, encoding="utf-8")
 
 
 def test_glob_relative(tmp_path, monkeypatch):
diff --git a/setuptools/tests/config/test_pyprojecttoml.py b/setuptools/tests/config/test_pyprojecttoml.py
index 6a40f3bfd7..abec68ab30 100644
--- a/setuptools/tests/config/test_pyprojecttoml.py
+++ b/setuptools/tests/config/test_pyprojecttoml.py
@@ -2,6 +2,7 @@
 from configparser import ConfigParser
 from inspect import cleandoc
 
+import jaraco.path
 import pytest
 import tomli_w
 from path import Path
@@ -82,25 +83,32 @@
 
 
 def create_example(path, pkg_root):
-    pyproject = path / "pyproject.toml"
-
-    files = [
-        f"{pkg_root}/pkg/__init__.py",
-        "_files/file.txt",
-    ]
-    if pkg_root != ".":  # flat-layout will raise error for multi-package dist
-        # Ensure namespaces are discovered
-        files.append(f"{pkg_root}/other/nested/__init__.py")
+    files = {
+        "pyproject.toml": EXAMPLE,
+        "README.md": "hello world",
+        "_files": {
+            "file.txt": "",
+        },
+    }
+    packages = {
+        "pkg": {
+            "__init__.py": "",
+            "mod.py": "class CustomSdist: pass",
+            "__version__.py": "VERSION = (3, 10)",
+            "__main__.py": "def exec(): print('hello')",
+        },
+    }
+
+    assert pkg_root  # Meta-test: cannot be empty string.
 
-    for file in files:
-        (path / file).parent.mkdir(exist_ok=True, parents=True)
-        (path / file).touch()
+    if pkg_root == ".":
+        files = {**files, **packages}
+        # skip other files: flat-layout will raise error for multi-package dist
+    else:
+        # Use this opportunity to ensure namespaces are discovered
+        files[pkg_root] = {**packages, "other": {"nested": {"__init__.py": ""}}}
 
-    pyproject.write_text(EXAMPLE)
-    (path / "README.md").write_text("hello world")
-    (path / f"{pkg_root}/pkg/mod.py").write_text("class CustomSdist: pass")
-    (path / f"{pkg_root}/pkg/__version__.py").write_text("VERSION = (3, 10)")
-    (path / f"{pkg_root}/pkg/__main__.py").write_text("def exec(): print('hello')")
+    jaraco.path.build(files, prefix=path)
 
 
 def verify_example(config, path, pkg_root):
@@ -174,7 +182,7 @@ class TestEntryPoints:
     def write_entry_points(self, tmp_path):
         entry_points = ConfigParser()
         entry_points.read_dict(ENTRY_POINTS)
-        with open(tmp_path / "entry-points.txt", "w") as f:
+        with open(tmp_path / "entry-points.txt", "w", encoding="utf-8") as f:
             entry_points.write(f)
 
     def pyproject(self, dynamic=None):
@@ -208,11 +216,13 @@ def test_dynamic(self, tmp_path):
         # Let's create a project example that has dynamic classifiers
         # coming from a txt file.
         create_example(tmp_path, "src")
-        classifiers = """\
-        Framework :: Flask
-        Programming Language :: Haskell
-        """
-        (tmp_path / "classifiers.txt").write_text(cleandoc(classifiers))
+        classifiers = cleandoc(
+            """
+            Framework :: Flask
+            Programming Language :: Haskell
+            """
+        )
+        (tmp_path / "classifiers.txt").write_text(classifiers, encoding="utf-8")
 
         pyproject = tmp_path / "pyproject.toml"
         config = read_configuration(pyproject, expand=False)
@@ -240,7 +250,7 @@ def test_dynamic_without_config(self, tmp_path):
         """
 
         pyproject = tmp_path / "pyproject.toml"
-        pyproject.write_text(cleandoc(config))
+        pyproject.write_text(cleandoc(config), encoding="utf-8")
         with pytest.raises(OptionError, match="No configuration .* .classifiers."):
             read_configuration(pyproject)
 
@@ -252,7 +262,7 @@ def test_dynamic_readme_from_setup_script_args(self, tmp_path):
         dynamic = ["readme"]
         """
         pyproject = tmp_path / "pyproject.toml"
-        pyproject.write_text(cleandoc(config))
+        pyproject.write_text(cleandoc(config), encoding="utf-8")
         dist = Distribution(attrs={"long_description": "42"})
         # No error should occur because of missing `readme`
         dist = apply_configuration(dist, pyproject)
@@ -270,7 +280,7 @@ def test_dynamic_without_file(self, tmp_path):
         """
 
         pyproject = tmp_path / "pyproject.toml"
-        pyproject.write_text(cleandoc(config))
+        pyproject.write_text(cleandoc(config), encoding="utf-8")
         with pytest.warns(UserWarning, match="File .*classifiers.txt. cannot be found"):
             expanded = read_configuration(pyproject)
         assert "classifiers" not in expanded["project"]
@@ -291,7 +301,7 @@ def test_dynamic_without_file(self, tmp_path):
 )
 def test_ignore_unrelated_config(tmp_path, example):
     pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text(cleandoc(example))
+    pyproject.write_text(cleandoc(example), encoding="utf-8")
 
     # Make sure no error is raised due to 3rd party configs in pyproject.toml
     assert read_configuration(pyproject) is not None
@@ -313,7 +323,7 @@ def test_ignore_unrelated_config(tmp_path, example):
 )
 def test_invalid_example(tmp_path, example, error_msg):
     pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text(cleandoc(example))
+    pyproject.write_text(cleandoc(example), encoding="utf-8")
 
     pattern = re.compile(f"invalid pyproject.toml.*{error_msg}.*", re.M | re.S)
     with pytest.raises(ValueError, match=pattern):
@@ -323,7 +333,7 @@ def test_invalid_example(tmp_path, example, error_msg):
 @pytest.mark.parametrize("config", ("", "[tool.something]\nvalue = 42"))
 def test_empty(tmp_path, config):
     pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text(config)
+    pyproject.write_text(config, encoding="utf-8")
 
     # Make sure no error is raised
     assert read_configuration(pyproject) == {}
@@ -335,7 +345,7 @@ def test_include_package_data_by_default(tmp_path, config):
     default.
     """
     pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text(config)
+    pyproject.write_text(config, encoding="utf-8")
 
     config = read_configuration(pyproject)
     assert config["tool"]["setuptools"]["include-package-data"] is True
@@ -347,10 +357,11 @@ def test_include_package_data_in_setuppy(tmp_path):
 
     See https://github.com/pypa/setuptools/issues/3197#issuecomment-1079023889
     """
-    pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text("[project]\nname = 'myproj'\nversion='42'\n")
-    setuppy = tmp_path / "setup.py"
-    setuppy.write_text("__import__('setuptools').setup(include_package_data=False)")
+    files = {
+        "pyproject.toml": "[project]\nname = 'myproj'\nversion='42'\n",
+        "setup.py": "__import__('setuptools').setup(include_package_data=False)",
+    }
+    jaraco.path.build(files, prefix=tmp_path)
 
     with Path(tmp_path):
         dist = distutils.core.run_setup("setup.py", {}, stop_after="config")
diff --git a/setuptools/tests/config/test_pyprojecttoml_dynamic_deps.py b/setuptools/tests/config/test_pyprojecttoml_dynamic_deps.py
index b6516227c0..37e5234a45 100644
--- a/setuptools/tests/config/test_pyprojecttoml_dynamic_deps.py
+++ b/setuptools/tests/config/test_pyprojecttoml_dynamic_deps.py
@@ -1,57 +1,59 @@
+from inspect import cleandoc
+
 import pytest
+from jaraco import path
 
 from setuptools.config.pyprojecttoml import apply_configuration
 from setuptools.dist import Distribution
-from setuptools.tests.textwrap import DALS
 
 
 def test_dynamic_dependencies(tmp_path):
-    (tmp_path / "requirements.txt").write_text("six\n  # comment\n")
-    pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text(
-        DALS(
+    files = {
+        "requirements.txt": "six\n  # comment\n",
+        "pyproject.toml": cleandoc(
             """
-    [project]
-    name = "myproj"
-    version = "1.0"
-    dynamic = ["dependencies"]
+            [project]
+            name = "myproj"
+            version = "1.0"
+            dynamic = ["dependencies"]
 
-    [build-system]
-    requires = ["setuptools", "wheel"]
-    build-backend = "setuptools.build_meta"
+            [build-system]
+            requires = ["setuptools", "wheel"]
+            build-backend = "setuptools.build_meta"
 
-    [tool.setuptools.dynamic.dependencies]
-    file = ["requirements.txt"]
-    """
-        )
-    )
+            [tool.setuptools.dynamic.dependencies]
+            file = ["requirements.txt"]
+            """
+        ),
+    }
+    path.build(files, prefix=tmp_path)
     dist = Distribution()
-    dist = apply_configuration(dist, pyproject)
+    dist = apply_configuration(dist, tmp_path / "pyproject.toml")
     assert dist.install_requires == ["six"]
 
 
 def test_dynamic_optional_dependencies(tmp_path):
-    (tmp_path / "requirements-docs.txt").write_text("sphinx\n  # comment\n")
-    pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text(
-        DALS(
+    files = {
+        "requirements-docs.txt": "sphinx\n  # comment\n",
+        "pyproject.toml": cleandoc(
             """
-    [project]
-    name = "myproj"
-    version = "1.0"
-    dynamic = ["optional-dependencies"]
+            [project]
+            name = "myproj"
+            version = "1.0"
+            dynamic = ["optional-dependencies"]
 
-    [tool.setuptools.dynamic.optional-dependencies.docs]
-    file = ["requirements-docs.txt"]
+            [tool.setuptools.dynamic.optional-dependencies.docs]
+            file = ["requirements-docs.txt"]
 
-    [build-system]
-    requires = ["setuptools", "wheel"]
-    build-backend = "setuptools.build_meta"
-    """
-        )
-    )
+            [build-system]
+            requires = ["setuptools", "wheel"]
+            build-backend = "setuptools.build_meta"
+            """
+        ),
+    }
+    path.build(files, prefix=tmp_path)
     dist = Distribution()
-    dist = apply_configuration(dist, pyproject)
+    dist = apply_configuration(dist, tmp_path / "pyproject.toml")
     assert dist.extras_require == {"docs": ["sphinx"]}
 
 
@@ -61,29 +63,32 @@ def test_mixed_dynamic_optional_dependencies(tmp_path):
     configurations in the case of fields containing sub-fields (groups),
     things would work out.
     """
-    (tmp_path / "requirements-images.txt").write_text("pillow~=42.0\n  # comment\n")
-    pyproject = tmp_path / "pyproject.toml"
-    pyproject.write_text(
-        DALS(
+    files = {
+        "requirements-images.txt": "pillow~=42.0\n  # comment\n",
+        "pyproject.toml": cleandoc(
             """
-    [project]
-    name = "myproj"
-    version = "1.0"
-    dynamic = ["optional-dependencies"]
+            [project]
+            name = "myproj"
+            version = "1.0"
+            dynamic = ["optional-dependencies"]
 
-    [project.optional-dependencies]
-    docs = ["sphinx"]
+            [project.optional-dependencies]
+            docs = ["sphinx"]
 
-    [tool.setuptools.dynamic.optional-dependencies.images]
-    file = ["requirements-images.txt"]
+            [tool.setuptools.dynamic.optional-dependencies.images]
+            file = ["requirements-images.txt"]
+
+            [build-system]
+            requires = ["setuptools", "wheel"]
+            build-backend = "setuptools.build_meta"
+            """
+        ),
+    }
+
+    path.build(files, prefix=tmp_path)
 
-    [build-system]
-    requires = ["setuptools", "wheel"]
-    build-backend = "setuptools.build_meta"
-    """
-        )
-    )
     # Test that the mix-and-match doesn't currently validate.
+    pyproject = tmp_path / "pyproject.toml"
     with pytest.raises(ValueError, match="project.optional-dependencies"):
         apply_configuration(Distribution(), pyproject)
 
diff --git a/setuptools/tests/config/test_setupcfg.py b/setuptools/tests/config/test_setupcfg.py
index 7f93858bd4..706e2d0ebe 100644
--- a/setuptools/tests/config/test_setupcfg.py
+++ b/setuptools/tests/config/test_setupcfg.py
@@ -904,7 +904,8 @@ def test_cmdclass(self, tmpdir):
         module_path = Path(tmpdir, "src/custom_build.py")  # auto discovery for src
         module_path.parent.mkdir(parents=True, exist_ok=True)
         module_path.write_text(
-            "from distutils.core import Command\n" "class CustomCmd(Command): pass\n"
+            "from distutils.core import Command\n" "class CustomCmd(Command): pass\n",
+            encoding="utf-8",
         )
 
         setup_cfg = """
diff --git a/setuptools/tests/integration/helpers.py b/setuptools/tests/integration/helpers.py
index 82cb36a2e4..615c43b2e0 100644
--- a/setuptools/tests/integration/helpers.py
+++ b/setuptools/tests/integration/helpers.py
@@ -17,6 +17,7 @@ def run(cmd, env=None):
         cmd,
         capture_output=True,
         text=True,
+        encoding="utf-8",
         env={**os.environ, **(env or {})},
         # ^-- allow overwriting instead of discarding the current env
     )
diff --git a/setuptools/tests/test_build_meta.py b/setuptools/tests/test_build_meta.py
index 9332781764..c2a1e6dc75 100644
--- a/setuptools/tests/test_build_meta.py
+++ b/setuptools/tests/test_build_meta.py
@@ -272,14 +272,14 @@ def test_build_with_existing_file_present(self, build_type, tmpdir_cwd):
                 [metadata]
                 name = foo
                 version = file: VERSION
-            """
+                """
             ),
             'pyproject.toml': DALS(
                 """
                 [build-system]
                 requires = ["setuptools", "wheel"]
                 build-backend = "setuptools.build_meta"
-            """
+                """
             ),
         }
 
@@ -296,7 +296,7 @@ def test_build_with_existing_file_present(self, build_type, tmpdir_cwd):
         first_result = build_method(dist_dir)
 
         # Change version.
-        with open("VERSION", "wt") as version_file:
+        with open("VERSION", "wt", encoding="utf-8") as version_file:
             version_file.write("0.0.2")
 
         # Build a *second* sdist/wheel.
@@ -306,7 +306,7 @@ def test_build_with_existing_file_present(self, build_type, tmpdir_cwd):
         assert first_result != second_result
 
         # And if rebuilding the exact same sdist/wheel?
-        open(os.path.join(dist_dir, second_result), 'w').close()
+        open(os.path.join(dist_dir, second_result), 'wb').close()
         third_result = build_method(dist_dir)
         assert third_result == second_result
         assert os.path.getsize(os.path.join(dist_dir, third_result)) > 0
@@ -568,9 +568,9 @@ def test_build_sdist_version_change(self, build_backend):
         if not os.path.exists(setup_loc):
             setup_loc = os.path.abspath("setup.cfg")
 
-        with open(setup_loc, 'rt') as file_handler:
+        with open(setup_loc, 'rt', encoding="utf-8") as file_handler:
             content = file_handler.read()
-        with open(setup_loc, 'wt') as file_handler:
+        with open(setup_loc, 'wt', encoding="utf-8") as file_handler:
             file_handler.write(content.replace("version='0.0.0'", "version='0.0.1'"))
 
         shutil.rmtree(sdist_into_directory)
diff --git a/setuptools/tests/test_build_py.py b/setuptools/tests/test_build_py.py
index 500a9ab6f3..4aa1fe68fa 100644
--- a/setuptools/tests/test_build_py.py
+++ b/setuptools/tests/test_build_py.py
@@ -49,7 +49,7 @@ def test_recursive_in_package_data_glob(tmpdir_cwd):
         )
     )
     os.makedirs('path/subpath/subsubpath')
-    open('path/subpath/subsubpath/data', 'w').close()
+    open('path/subpath/subsubpath/data', 'wb').close()
 
     dist.parse_command_line()
     dist.run_commands()
@@ -77,8 +77,8 @@ def test_read_only(tmpdir_cwd):
         )
     )
     os.makedirs('pkg')
-    open('pkg/__init__.py', 'w').close()
-    open('pkg/data.dat', 'w').close()
+    open('pkg/__init__.py', 'wb').close()
+    open('pkg/data.dat', 'wb').close()
     os.chmod('pkg/__init__.py', stat.S_IREAD)
     os.chmod('pkg/data.dat', stat.S_IREAD)
     dist.parse_command_line()
@@ -108,8 +108,8 @@ def test_executable_data(tmpdir_cwd):
         )
     )
     os.makedirs('pkg')
-    open('pkg/__init__.py', 'w').close()
-    open('pkg/run-me', 'w').close()
+    open('pkg/__init__.py', 'wb').close()
+    open('pkg/run-me', 'wb').close()
     os.chmod('pkg/run-me', 0o700)
 
     dist.parse_command_line()
diff --git a/setuptools/tests/test_config_discovery.py b/setuptools/tests/test_config_discovery.py
index ef2979d4f5..72772caebf 100644
--- a/setuptools/tests/test_config_discovery.py
+++ b/setuptools/tests/test_config_discovery.py
@@ -176,11 +176,11 @@ def test_purposefully_empty(self, tmp_path, config_file, param, circumstance):
         else:
             # Make sure build works with or without setup.cfg
             pyproject = self.PURPOSEFULLY_EMPY["template-pyproject.toml"]
-            (tmp_path / "pyproject.toml").write_text(pyproject)
+            (tmp_path / "pyproject.toml").write_text(pyproject, encoding="utf-8")
             template_param = param
 
         config = self.PURPOSEFULLY_EMPY[config_file].format(param=template_param)
-        (tmp_path / config_file).write_text(config)
+        (tmp_path / config_file).write_text(config, encoding="utf-8")
 
         dist = _get_dist(tmp_path, {})
         # When either parameter package or py_modules is an empty list,
@@ -292,11 +292,13 @@ class TestWithAttrDirective:
     def test_setupcfg_metadata(self, tmp_path, folder, opts):
         files = [f"{folder}/pkg/__init__.py", "setup.cfg"]
         _populate_project_dir(tmp_path, files, opts)
-        (tmp_path / folder / "pkg/__init__.py").write_text("version = 42")
-        (tmp_path / "setup.cfg").write_text(
-            "[metadata]\nversion = attr: pkg.version\n"
-            + (tmp_path / "setup.cfg").read_text()
-        )
+
+        config = (tmp_path / "setup.cfg").read_text(encoding="utf-8")
+        overwrite = {
+            folder: {"pkg": {"__init__.py": "version = 42"}},
+            "setup.cfg": "[metadata]\nversion = attr: pkg.version\n" + config,
+        }
+        jaraco.path.build(overwrite, prefix=tmp_path)
 
         dist = _get_dist(tmp_path, {})
         assert dist.get_name() == "pkg"
@@ -312,11 +314,16 @@ def test_setupcfg_metadata(self, tmp_path, folder, opts):
 
     def test_pyproject_metadata(self, tmp_path):
         _populate_project_dir(tmp_path, ["src/pkg/__init__.py"], {})
-        (tmp_path / "src/pkg/__init__.py").write_text("version = 42")
-        (tmp_path / "pyproject.toml").write_text(
-            "[project]\nname = 'pkg'\ndynamic = ['version']\n"
-            "[tool.setuptools.dynamic]\nversion = {attr = 'pkg.version'}\n"
-        )
+
+        overwrite = {
+            "src": {"pkg": {"__init__.py": "version = 42"}},
+            "pyproject.toml": (
+                "[project]\nname = 'pkg'\ndynamic = ['version']\n"
+                "[tool.setuptools.dynamic]\nversion = {attr = 'pkg.version'}\n"
+            ),
+        }
+        jaraco.path.build(overwrite, prefix=tmp_path)
+
         dist = _get_dist(tmp_path, {})
         assert dist.get_version() == "42"
         assert dist.package_dir == {"": "src"}
@@ -354,7 +361,7 @@ def _simulate_package_with_extension(self, tmp_path):
             ]
             setup(ext_modules=ext_modules)
         """
-        (tmp_path / "setup.py").write_text(DALS(setup_script))
+        (tmp_path / "setup.py").write_text(DALS(setup_script), encoding="utf-8")
 
     def test_skip_discovery_with_setupcfg_metadata(self, tmp_path):
         """Ensure that auto-discovery is not triggered when the project is based on
@@ -367,14 +374,14 @@ def test_skip_discovery_with_setupcfg_metadata(self, tmp_path):
             requires = []
             build-backend = 'setuptools.build_meta'
         """
-        (tmp_path / "pyproject.toml").write_text(DALS(pyproject))
+        (tmp_path / "pyproject.toml").write_text(DALS(pyproject), encoding="utf-8")
 
         setupcfg = """
             [metadata]
             name = proj
             version = 42
         """
-        (tmp_path / "setup.cfg").write_text(DALS(setupcfg))
+        (tmp_path / "setup.cfg").write_text(DALS(setupcfg), encoding="utf-8")
 
         dist = _get_dist(tmp_path, {})
         assert dist.get_name() == "proj"
@@ -399,7 +406,7 @@ def test_dont_skip_discovery_with_pyproject_metadata(self, tmp_path):
             name = 'proj'
             version = '42'
         """
-        (tmp_path / "pyproject.toml").write_text(DALS(pyproject))
+        (tmp_path / "pyproject.toml").write_text(DALS(pyproject), encoding="utf-8")
         with pytest.raises(PackageDiscoveryError, match="multiple (packages|modules)"):
             _get_dist(tmp_path, {})
 
@@ -416,7 +423,7 @@ def _simulate_package_with_data_files(self, tmp_path, src_root):
         manifest = """
             global-include *.py *.txt
         """
-        (tmp_path / "MANIFEST.in").write_text(DALS(manifest))
+        (tmp_path / "MANIFEST.in").write_text(DALS(manifest), encoding="utf-8")
 
     EXAMPLE_SETUPCFG = """
     [metadata]
@@ -564,9 +571,12 @@ def _populate_project_dir(root, files, options):
     # NOTE: Currently pypa/build will refuse to build the project if no
     # `pyproject.toml` or `setup.py` is found. So it is impossible to do
     # completely "config-less" projects.
-    (root / "setup.py").write_text("import setuptools\nsetuptools.setup()")
-    (root / "README.md").write_text("# Example Package")
-    (root / "LICENSE").write_text("Copyright (c) 2018")
+    basic = {
+        "setup.py": "import setuptools\nsetuptools.setup()",
+        "README.md": "# Example Package",
+        "LICENSE": "Copyright (c) 2018",
+    }
+    jaraco.path.build(basic, prefix=root)
     _write_setupcfg(root, options)
     paths = (root / f for f in files)
     for path in paths:
@@ -591,10 +601,10 @@ def _write_setupcfg(root, options):
             setupcfg["options"][key] = "\n" + str_value
         else:
             setupcfg["options"][key] = str(value)
-    with open(root / "setup.cfg", "w") as f:
+    with open(root / "setup.cfg", "w", encoding="utf-8") as f:
         setupcfg.write(f)
     print("~~~~~ setup.cfg ~~~~~")
-    print((root / "setup.cfg").read_text())
+    print((root / "setup.cfg").read_text(encoding="utf-8"))
 
 
 def _run_build(path, *flags):
diff --git a/setuptools/tests/test_dist_info.py b/setuptools/tests/test_dist_info.py
index ad6cebad0b..c6fe97e2ba 100644
--- a/setuptools/tests/test_dist_info.py
+++ b/setuptools/tests/test_dist_info.py
@@ -198,7 +198,8 @@ def run_command_inner(*cmd, **kwargs):
         "stderr": subprocess.STDOUT,
         "stdout": subprocess.PIPE,
         "text": True,
-        'check': True,
+        "encoding": "utf-8",
+        "check": True,
         **kwargs,
     }
     cmd = [sys.executable, "-c", "__import__('setuptools').setup()", *map(str, cmd)]
diff --git a/setuptools/tests/test_distutils_adoption.py b/setuptools/tests/test_distutils_adoption.py
index eb7feba637..74883d2199 100644
--- a/setuptools/tests/test_distutils_adoption.py
+++ b/setuptools/tests/test_distutils_adoption.py
@@ -8,6 +8,8 @@
 
 IS_PYPY = '__pypy__' in sys.builtin_module_names
 
+_TEXT_KWARGS = {"text": True, "encoding": "utf-8"}  # For subprocess.run
+
 
 def win_sr(env):
     """
@@ -24,7 +26,7 @@ def win_sr(env):
 def find_distutils(venv, imports='distutils', env=None, **kwargs):
     py_cmd = 'import {imports}; print(distutils.__file__)'.format(**locals())
     cmd = ['python', '-c', py_cmd]
-    return venv.run(cmd, env=win_sr(env), text=True, **kwargs)
+    return venv.run(cmd, env=win_sr(env), **_TEXT_KWARGS, **kwargs)
 
 
 def count_meta_path(venv, env=None):
@@ -36,7 +38,7 @@ def count_meta_path(venv, env=None):
         """
     )
     cmd = ['python', '-c', py_cmd]
-    return int(venv.run(cmd, env=win_sr(env), text=True))
+    return int(venv.run(cmd, env=win_sr(env), **_TEXT_KWARGS))
 
 
 skip_without_stdlib_distutils = pytest.mark.skipif(
@@ -82,7 +84,7 @@ def test_pip_import(venv):
     Regression test for #3002.
     """
     cmd = ['python', '-c', 'import pip']
-    venv.run(cmd, text=True)
+    venv.run(cmd, **_TEXT_KWARGS)
 
 
 def test_distutils_has_origin():
@@ -130,7 +132,7 @@ def test_modules_are_not_duplicated_on_import(
     env = dict(SETUPTOOLS_USE_DISTUTILS=distutils_version)
     script = ENSURE_IMPORTS_ARE_NOT_DUPLICATED.format(imported_module=imported_module)
     cmd = ['python', '-c', script]
-    output = venv.run(cmd, env=win_sr(env), text=True).strip()
+    output = venv.run(cmd, env=win_sr(env), **_TEXT_KWARGS).strip()
     assert output == "success"
 
 
@@ -154,5 +156,5 @@ def test_modules_are_not_duplicated_on_import(
 def test_log_module_is_not_duplicated_on_import(distutils_version, tmpdir_cwd, venv):
     env = dict(SETUPTOOLS_USE_DISTUTILS=distutils_version)
     cmd = ['python', '-c', ENSURE_LOG_IMPORT_IS_NOT_DUPLICATED]
-    output = venv.run(cmd, env=win_sr(env), text=True).strip()
+    output = venv.run(cmd, env=win_sr(env), **_TEXT_KWARGS).strip()
     assert output == "success"
diff --git a/setuptools/tests/test_easy_install.py b/setuptools/tests/test_easy_install.py
index a001ae27c5..950cb23d21 100644
--- a/setuptools/tests/test_easy_install.py
+++ b/setuptools/tests/test_easy_install.py
@@ -467,7 +467,7 @@ def distutils_package():
         'from distutils.core import setup',
     )
     with contexts.tempdir(cd=os.chdir):
-        with open('setup.py', 'w') as f:
+        with open('setup.py', 'w', encoding="utf-8") as f:
             f.write(distutils_setup_py)
         yield
 
@@ -530,6 +530,7 @@ def test_setup_install_includes_dependencies(self, tmp_path, mock_index):
             stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT,
             text=True,
+            encoding="utf-8",
         )
         assert cp.returncode != 0
         try:
@@ -784,7 +785,7 @@ def test_setup_requires_honors_pip_env(self, mock_index, monkeypatch):
                     setup_attrs=dict(dependency_links=[]),
                 )
                 test_setup_cfg = os.path.join(test_pkg, 'setup.cfg')
-                with open(test_setup_cfg, 'w') as fp:
+                with open(test_setup_cfg, 'w', encoding="utf-8") as fp:
                     fp.write(
                         DALS(
                             """
@@ -918,7 +919,7 @@ def test_setup_requires_with_find_links_in_setup_cfg(
                 test_setup_py = os.path.join(test_pkg, 'setup.py')
                 test_setup_cfg = os.path.join(test_pkg, 'setup.cfg')
                 os.mkdir(test_pkg)
-                with open(test_setup_py, 'w') as fp:
+                with open(test_setup_py, 'w', encoding="utf-8") as fp:
                     if with_dependency_links_in_setup_py:
                         dependency_links = [os.path.join(temp_dir, 'links')]
                     else:
@@ -932,7 +933,7 @@ def test_setup_requires_with_find_links_in_setup_cfg(
                         """
                         ).format(dependency_links=dependency_links)
                     )
-                with open(test_setup_cfg, 'w') as fp:
+                with open(test_setup_cfg, 'w', encoding="utf-8") as fp:
                     fp.write(
                         DALS(
                             """
@@ -984,7 +985,7 @@ def test_setup_requires_with_transitive_extra_dependency(self, monkeypatch):
                 test_pkg = os.path.join(temp_dir, 'test_pkg')
                 test_setup_py = os.path.join(test_pkg, 'setup.py')
                 os.mkdir(test_pkg)
-                with open(test_setup_py, 'w') as fp:
+                with open(test_setup_py, 'w', encoding="utf-8") as fp:
                     fp.write(
                         DALS(
                             """
@@ -1068,7 +1069,7 @@ class epcmd(build_py):
                 test_pkg = os.path.join(temp_dir, 'test_pkg')
                 test_setup_py = os.path.join(test_pkg, 'setup.py')
                 os.mkdir(test_pkg)
-                with open(test_setup_py, 'w') as fp:
+                with open(test_setup_py, 'w', encoding="utf-8") as fp:
                     fp.write(
                         DALS(
                             """
@@ -1244,7 +1245,7 @@ def create_setup_requires_package(
         )
     else:
         test_setup_cfg_contents = ''
-    with open(os.path.join(test_pkg, 'setup.cfg'), 'w') as f:
+    with open(os.path.join(test_pkg, 'setup.cfg'), 'w', encoding="utf-8") as f:
         f.write(test_setup_cfg_contents)
 
     # setup.py
@@ -1255,7 +1256,7 @@ def create_setup_requires_package(
             setuptools.setup(**%r)
         """
         )
-    with open(os.path.join(test_pkg, 'setup.py'), 'w') as f:
+    with open(os.path.join(test_pkg, 'setup.py'), 'w', encoding="utf-8") as f:
         f.write(setup_py_template % test_setup_attrs)
 
     foobar_path = os.path.join(path, '%s-%s.tar.gz' % (distname, version))
diff --git a/setuptools/tests/test_editable_install.py b/setuptools/tests/test_editable_install.py
index df85699586..1df09fd256 100644
--- a/setuptools/tests/test_editable_install.py
+++ b/setuptools/tests/test_editable_install.py
@@ -144,8 +144,8 @@ def test_editable_with_pyproject(tmp_path, venv, files, editable_opts):
     cmd = [venv.exe(), "-m", "mypkg"]
     assert subprocess.check_output(cmd).strip() == b"3.14159.post0 Hello World"
 
-    (project / "src/mypkg/data.txt").write_text("foobar")
-    (project / "src/mypkg/mod.py").write_text("x = 42")
+    (project / "src/mypkg/data.txt").write_text("foobar", encoding="utf-8")
+    (project / "src/mypkg/mod.py").write_text("x = 42", encoding="utf-8")
     assert subprocess.check_output(cmd).strip() == b"3.14159.post0 foobar 42"
 
 
diff --git a/setuptools/tests/test_egg_info.py b/setuptools/tests/test_egg_info.py
index ba019dc79d..a4b0ecf398 100644
--- a/setuptools/tests/test_egg_info.py
+++ b/setuptools/tests/test_egg_info.py
@@ -94,7 +94,7 @@ def test_egg_info_save_version_info_setup_empty(self, tmpdir_cwd, env):
         ei.initialize_options()
         ei.save_version_info(setup_cfg)
 
-        with open(setup_cfg, 'r') as f:
+        with open(setup_cfg, 'r', encoding="utf-8") as f:
             content = f.read()
 
         assert '[egg_info]' in content
@@ -139,7 +139,7 @@ def test_egg_info_save_version_info_setup_defaults(self, tmpdir_cwd, env):
         ei.initialize_options()
         ei.save_version_info(setup_cfg)
 
-        with open(setup_cfg, 'r') as f:
+        with open(setup_cfg, 'r', encoding="utf-8") as f:
             content = f.read()
 
         assert '[egg_info]' in content
@@ -251,7 +251,7 @@ def test_manifest_template_is_read(self, tmpdir_cwd, env):
         self._run_egg_info_command(tmpdir_cwd, env)
         egg_info_dir = os.path.join('.', 'foo.egg-info')
         sources_txt = os.path.join(egg_info_dir, 'SOURCES.txt')
-        with open(sources_txt) as f:
+        with open(sources_txt, encoding="utf-8") as f:
             assert 'docs/usage.rst' in f.read().split('\n')
 
     def _setup_script_with_requires(self, requires, use_setup_cfg=False):
@@ -492,7 +492,7 @@ def test_requires(
         egg_info_dir = os.path.join('.', 'foo.egg-info')
         requires_txt = os.path.join(egg_info_dir, 'requires.txt')
         if os.path.exists(requires_txt):
-            with open(requires_txt) as fp:
+            with open(requires_txt, encoding="utf-8") as fp:
                 install_requires = fp.read()
         else:
             install_requires = ''
@@ -538,8 +538,8 @@ def test_provides_extra(self, tmpdir_cwd, env):
             env=environ,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         assert 'Provides-Extra: foobar' in pkg_info_lines
         assert 'Metadata-Version: 2.1' in pkg_info_lines
 
@@ -557,8 +557,8 @@ def test_doesnt_provides_extra(self, tmpdir_cwd, env):
             env=environ,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_text = pkginfo_file.read()
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_text = fp.read()
         assert 'Provides-Extra:' not in pkg_info_text
 
     @pytest.mark.parametrize(
@@ -636,8 +636,7 @@ def test_setup_cfg_license_file(self, tmpdir_cwd, env, files, license_in_sources
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
 
-        with open(os.path.join(egg_info_dir, 'SOURCES.txt')) as sources_file:
-            sources_text = sources_file.read()
+        sources_text = Path(egg_info_dir, "SOURCES.txt").read_text(encoding="utf-8")
 
         if license_in_sources:
             assert 'LICENSE' in sources_text
@@ -849,8 +848,8 @@ def test_setup_cfg_license_files(
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
 
-        with open(os.path.join(egg_info_dir, 'SOURCES.txt')) as sources_file:
-            sources_lines = list(line.strip() for line in sources_file)
+        sources_text = Path(egg_info_dir, "SOURCES.txt").read_text(encoding="utf-8")
+        sources_lines = [line.strip() for line in sources_text.splitlines()]
 
         for lf in incl_licenses:
             assert sources_lines.count(lf) == 1
@@ -1033,8 +1032,8 @@ def test_setup_cfg_license_file_license_files(
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
 
-        with open(os.path.join(egg_info_dir, 'SOURCES.txt')) as sources_file:
-            sources_lines = list(line.strip() for line in sources_file)
+        sources_text = Path(egg_info_dir, "SOURCES.txt").read_text(encoding="utf-8")
+        sources_lines = [line.strip() for line in sources_text.splitlines()]
 
         for lf in incl_licenses:
             assert sources_lines.count(lf) == 1
@@ -1065,8 +1064,8 @@ def test_license_file_attr_pkg_info(self, tmpdir_cwd, env):
             pypath=os.pathsep.join([env.paths['lib'], str(tmpdir_cwd)]),
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         license_file_lines = [
             line for line in pkg_info_lines if line.startswith('License-File:')
         ]
@@ -1086,8 +1085,8 @@ def test_metadata_version(self, tmpdir_cwd, env):
             data_stream=1,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         # Update metadata version if changed
         assert self._extract_mv_version(pkg_info_lines) == (2, 1)
 
@@ -1112,8 +1111,8 @@ def test_long_description_content_type(self, tmpdir_cwd, env):
             env=environ,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         expected_line = 'Description-Content-Type: text/markdown'
         assert expected_line in pkg_info_lines
         assert 'Metadata-Version: 2.1' in pkg_info_lines
@@ -1133,8 +1132,8 @@ def test_long_description(self, tmpdir_cwd, env):
             data_stream=1,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         assert 'Metadata-Version: 2.1' in pkg_info_lines
         assert '' == pkg_info_lines[-1]  # last line should be empty
         long_desc_lines = pkg_info_lines[pkg_info_lines.index('') :]
@@ -1165,8 +1164,8 @@ def test_project_urls(self, tmpdir_cwd, env):
             env=environ,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         expected_line = 'Project-URL: Link One, https://example.com/one/'
         assert expected_line in pkg_info_lines
         expected_line = 'Project-URL: Link Two, https://example.com/two/'
@@ -1182,8 +1181,8 @@ def test_license(self, tmpdir_cwd, env):
             data_stream=1,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         assert 'License: MIT' in pkg_info_lines
 
     def test_license_escape(self, tmpdir_cwd, env):
@@ -1197,8 +1196,8 @@ def test_license_escape(self, tmpdir_cwd, env):
             data_stream=1,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
 
         assert 'License: This is a long license text ' in pkg_info_lines
         assert '        over multiple lines' in pkg_info_lines
@@ -1216,8 +1215,8 @@ def test_python_requires_egg_info(self, tmpdir_cwd, env):
             env=environ,
         )
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         assert 'Requires-Python: >=2.7.12' in pkg_info_lines
         assert self._extract_mv_version(pkg_info_lines) >= (1, 2)
 
@@ -1240,7 +1239,7 @@ def test_egg_info_includes_setup_py(self, tmpdir_cwd):
 
         assert 'setup.py' in egg_info_instance.filelist.files
 
-        with open(egg_info_instance.egg_info + "/SOURCES.txt") as f:
+        with open(egg_info_instance.egg_info + "/SOURCES.txt", encoding="utf-8") as f:
             sources = f.read().split('\n')
             assert 'setup.py' in sources
 
@@ -1277,8 +1276,8 @@ def test_egg_info_tag_only_once(self, tmpdir_cwd, env):
         })
         self._run_egg_info_command(tmpdir_cwd, env)
         egg_info_dir = os.path.join('.', 'foo.egg-info')
-        with open(os.path.join(egg_info_dir, 'PKG-INFO')) as pkginfo_file:
-            pkg_info_lines = pkginfo_file.read().split('\n')
+        with open(os.path.join(egg_info_dir, 'PKG-INFO'), encoding="utf-8") as fp:
+            pkg_info_lines = fp.read().split('\n')
         assert 'Version: 0.0.0.dev0' in pkg_info_lines
 
 
diff --git a/setuptools/tests/test_find_packages.py b/setuptools/tests/test_find_packages.py
index cb1900df3d..4fefd3dccf 100644
--- a/setuptools/tests/test_find_packages.py
+++ b/setuptools/tests/test_find_packages.py
@@ -72,8 +72,7 @@ def _mkdir(self, path, parent_dir=None):
     def _touch(self, path, dir_=None):
         if dir_:
             path = os.path.join(dir_, path)
-        fp = open(path, 'w')
-        fp.close()
+        open(path, 'wb').close()
         return path
 
     def test_regular_package(self):
diff --git a/setuptools/tests/test_install_scripts.py b/setuptools/tests/test_install_scripts.py
index a783459730..595b7ade67 100644
--- a/setuptools/tests/test_install_scripts.py
+++ b/setuptools/tests/test_install_scripts.py
@@ -41,7 +41,7 @@ def test_sys_executable_escaping_unix(self, tmpdir, monkeypatch):
         monkeypatch.setattr('sys.executable', self.unix_exe)
         with tmpdir.as_cwd():
             self._run_install_scripts(str(tmpdir))
-            with open(str(tmpdir.join('foo')), 'r') as f:
+            with open(str(tmpdir.join('foo')), 'r', encoding="utf-8") as f:
                 actual = f.readline()
         assert actual == expected
 
@@ -55,7 +55,7 @@ def test_sys_executable_escaping_win32(self, tmpdir, monkeypatch):
         monkeypatch.setattr('sys.executable', self.win32_exe)
         with tmpdir.as_cwd():
             self._run_install_scripts(str(tmpdir))
-            with open(str(tmpdir.join('foo-script.py')), 'r') as f:
+            with open(str(tmpdir.join('foo-script.py')), 'r', encoding="utf-8") as f:
                 actual = f.readline()
         assert actual == expected
 
@@ -69,7 +69,7 @@ def test_executable_with_spaces_escaping_unix(self, tmpdir):
         expected = '#!%s\n' % self.unix_spaces_exe
         with tmpdir.as_cwd():
             self._run_install_scripts(str(tmpdir), self.unix_spaces_exe)
-            with open(str(tmpdir.join('foo')), 'r') as f:
+            with open(str(tmpdir.join('foo')), 'r', encoding="utf-8") as f:
                 actual = f.readline()
         assert actual == expected
 
@@ -83,6 +83,6 @@ def test_executable_arg_escaping_win32(self, tmpdir):
         expected = '#!"%s"\n' % self.win32_exe
         with tmpdir.as_cwd():
             self._run_install_scripts(str(tmpdir), '"' + self.win32_exe + '"')
-            with open(str(tmpdir.join('foo-script.py')), 'r') as f:
+            with open(str(tmpdir.join('foo-script.py')), 'r', encoding="utf-8") as f:
                 actual = f.readline()
         assert actual == expected
diff --git a/setuptools/tests/test_logging.py b/setuptools/tests/test_logging.py
index 7a9a33f1ea..cf89b3bd00 100644
--- a/setuptools/tests/test_logging.py
+++ b/setuptools/tests/test_logging.py
@@ -33,7 +33,7 @@ def test_verbosity_level(tmp_path, monkeypatch, flag, expected_level):
     assert logging.getLevelName(unset_log_level) == "NOTSET"
 
     setup_script = tmp_path / "setup.py"
-    setup_script.write_text(setup_py)
+    setup_script.write_text(setup_py, encoding="utf-8")
     dist = distutils.core.run_setup(setup_script, stop_after="init")
     dist.script_args = [flag, "sdist"]
     dist.parse_command_line()  # <- where the log level is set
diff --git a/setuptools/tests/test_manifest.py b/setuptools/tests/test_manifest.py
index 16fa2c2460..6911b0224c 100644
--- a/setuptools/tests/test_manifest.py
+++ b/setuptools/tests/test_manifest.py
@@ -54,7 +54,7 @@ def quiet():
 
 
 def touch(filename):
-    open(filename, 'w').close()
+    open(filename, 'wb').close()
 
 
 # The set of files always in the manifest, including all files in the
@@ -174,7 +174,7 @@ class TestManifestTest(TempDirTestCase):
     def setup_method(self, method):
         super().setup_method(method)
 
-        f = open(os.path.join(self.temp_dir, 'setup.py'), 'w')
+        f = open(os.path.join(self.temp_dir, 'setup.py'), 'w', encoding="utf-8")
         f.write(SETUP_PY)
         f.close()
         """
@@ -212,7 +212,8 @@ def setup_method(self, method):
 
     def make_manifest(self, contents):
         """Write a MANIFEST.in."""
-        with open(os.path.join(self.temp_dir, 'MANIFEST.in'), 'w') as f:
+        manifest = os.path.join(self.temp_dir, 'MANIFEST.in')
+        with open(manifest, 'w', encoding="utf-8") as f:
             f.write(DALS(contents))
 
     def get_files(self):
@@ -369,7 +370,7 @@ def make_files(self, files):
             file = os.path.join(self.temp_dir, file)
             dirname, basename = os.path.split(file)
             os.makedirs(dirname, exist_ok=True)
-            open(file, 'w').close()
+            touch(file)
 
     def test_process_template_line(self):
         # testing  all MANIFEST.in template patterns
diff --git a/setuptools/tests/test_packageindex.py b/setuptools/tests/test_packageindex.py
index 41b96614f8..93474ae5af 100644
--- a/setuptools/tests/test_packageindex.py
+++ b/setuptools/tests/test_packageindex.py
@@ -2,12 +2,12 @@
 import urllib.request
 import urllib.error
 import http.client
+from inspect import cleandoc
 from unittest import mock
 
 import pytest
 
 import setuptools.package_index
-from .textwrap import DALS
 
 
 class TestPackageIndex:
@@ -257,14 +257,15 @@ class TestPyPIConfig:
     def test_percent_in_password(self, tmp_home_dir):
         pypirc = tmp_home_dir / '.pypirc'
         pypirc.write_text(
-            DALS(
+            cleandoc(
                 """
-            [pypi]
-            repository=https://pypi.org
-            username=jaraco
-            password=pity%
-        """
-            )
+                [pypi]
+                repository=https://pypi.org
+                username=jaraco
+                password=pity%
+                """
+            ),
+            encoding="utf-8",
         )
         cfg = setuptools.package_index.PyPIConfig()
         cred = cfg.creds_by_repository['https://pypi.org']
diff --git a/setuptools/tests/test_sandbox.py b/setuptools/tests/test_sandbox.py
index 9b4937e213..f666615d99 100644
--- a/setuptools/tests/test_sandbox.py
+++ b/setuptools/tests/test_sandbox.py
@@ -17,7 +17,7 @@ def test_devnull(self, tmpdir):
     @staticmethod
     def _file_writer(path):
         def do_write():
-            with open(path, 'w') as f:
+            with open(path, 'w', encoding="utf-8") as f:
                 f.write('xxx')
 
         return do_write
@@ -114,7 +114,7 @@ def test_sandbox_violation_raised_hiding_setuptools(self, tmpdir):
 
         def write_file():
             "Trigger a SandboxViolation by writing outside the sandbox"
-            with open('/etc/foo', 'w'):
+            with open('/etc/foo', 'w', encoding="utf-8"):
                 pass
 
         with pytest.raises(setuptools.sandbox.SandboxViolation) as caught:
@@ -126,8 +126,9 @@ def write_file():
         cmd, args, kwargs = caught.value.args
         assert cmd == 'open'
         assert args == ('/etc/foo', 'w')
-        assert kwargs == {}
+        assert kwargs == {"encoding": "utf-8"}
 
         msg = str(caught.value)
         assert 'open' in msg
         assert "('/etc/foo', 'w')" in msg
+        assert "{'encoding': 'utf-8'}" in msg
diff --git a/setuptools/tests/test_sdist.py b/setuptools/tests/test_sdist.py
index 5d597709ed..387ec3bebf 100644
--- a/setuptools/tests/test_sdist.py
+++ b/setuptools/tests/test_sdist.py
@@ -10,7 +10,6 @@
 import logging
 import distutils
 from inspect import cleandoc
-from pathlib import Path
 from unittest import mock
 
 import pytest
@@ -116,9 +115,7 @@ def latin1_fail():
 
 
 def touch(path):
-    if isinstance(path, str):
-        path = Path(path)
-    path.write_text('', encoding='utf-8')
+    open(path, 'wb').close()
     return path
 
 
@@ -386,7 +383,7 @@ def test_setup_py_missing(self):
         assert 'setup.py' not in manifest
 
     def test_setup_py_excluded(self):
-        with open("MANIFEST.in", "w") as manifest_file:
+        with open("MANIFEST.in", "w", encoding="utf-8") as manifest_file:
             manifest_file.write("exclude setup.py")
 
         dist = Distribution(SETUP_ATTRS)
@@ -441,7 +438,7 @@ def test_manifest_is_written_with_utf8_encoding(self):
         filename = os.path.join('sdist_test', 'smörbröd.py')
 
         # Must create the file or it will get stripped.
-        open(filename, 'w').close()
+        touch(filename)
 
         # Add UTF-8 filename and write manifest
         with quiet():
@@ -469,7 +466,7 @@ def test_write_manifest_allows_utf8_filenames(self):
         filename = os.path.join(b'sdist_test', Filenames.utf_8)
 
         # Must touch the file or risk removal
-        open(filename, "w").close()
+        touch(filename)
 
         # Add filename and write manifest
         with quiet():
@@ -546,7 +543,7 @@ def test_manifest_is_read_with_utf8_encoding(self):
         manifest.close()
 
         # The file must exist to be included in the filelist
-        open(filename, 'w').close()
+        touch(filename)
 
         # Re-read manifest
         cmd.filelist.files = []
@@ -577,7 +574,7 @@ def test_read_manifest_skips_non_utf8_filenames(self):
         manifest.close()
 
         # The file must exist to be included in the filelist
-        open(filename, 'w').close()
+        touch(filename)
 
         # Re-read manifest
         cmd.filelist.files = []
@@ -598,7 +595,7 @@ def test_sdist_with_utf8_encoded_filename(self):
         cmd.ensure_finalized()
 
         filename = os.path.join(b'sdist_test', Filenames.utf_8)
-        open(filename, 'w').close()
+        touch(filename)
 
         with quiet():
             cmd.run()
@@ -639,7 +636,7 @@ def test_sdist_with_latin1_encoded_filename(self):
 
         # Latin-1 filename
         filename = os.path.join(b'sdist_test', Filenames.latin_1)
-        open(filename, 'w').close()
+        touch(filename)
         assert os.path.isfile(filename)
 
         with quiet():
@@ -736,7 +733,7 @@ def test_pyproject_toml_excluded(self, source_dir):
         Check that pyproject.toml can excluded even if present
         """
         touch(source_dir / 'pyproject.toml')
-        with open('MANIFEST.in', 'w') as mts:
+        with open('MANIFEST.in', 'w', encoding="utf-8") as mts:
             print('exclude pyproject.toml', file=mts)
         dist = Distribution(SETUP_ATTRS)
         dist.script_name = 'setup.py'