De-dup oldest Python and attrs

noxfile.py

@@ -12,14 +12,28 @@
 nox.options.error_on_external_run = True
 
 
-MATCH_PYTHON = re.compile(r"\s+python\: \"(\d\.\d\d)\"").match
-# Avoid dependency on a YAML lib using a questionable hack.
+# Avoid dependencies on YAML / TOML libs using a questionable hacks.
+match_docs_python = re.compile(r"\s+python\: \"(\d\.\d\d)\"").fullmatch
 for line in Path(".readthedocs.yaml").read_text().splitlines():
-    m = MATCH_PYTHON(line)
+    m = match_docs_python(line)
     if m:
         DOCS_PYTHON = m.group(1)
         break
 
+match_oldest_python = re.compile(
+    r"requires-python = \">=(\d\.\d+)\""
+).fullmatch
+match_oldest_attrs = re.compile(r".*\"attrs>=(\d+\.\d+\.\d+)\",.*").match
+for line in Path("pyproject.toml").read_text().splitlines():
+    m = match_oldest_python(line)
+    if m:
+        OLDEST_PYTHON = m.group(1)
+
+    m = match_oldest_attrs(line)
+    if m:
+        OLDEST_ATTRS = m.group(1)
+        break  # dependencies always come after requires-python
+
 
 @nox.session
 def pre_commit(session: nox.Session) -> None:
@@ -49,10 +63,9 @@ def tests(session: nox.Session) -> None:
     session.run("pytest", *session.posargs)
 
 
-@nox.session(python="3.7", tags=["tests"])
+@nox.session(python=OLDEST_PYTHON, tags=["tests"])
 def tests_oldest_attrs(session: nox.Session) -> None:
-    # Keeps attrs pin in sync with pyproject.toml/dependencies.
-    session.install(".[tests]", "attrs==17.4.0")
+    session.install(".[tests]", f"attrs=={OLDEST_ATTRS}")
 
     _cov(session)
 

pyproject.toml

@@ -14,7 +14,6 @@ dynamic = ["readme", "version"]
 name = "environ-config"
 description = "Boilerplate-free configuration with env variables."
 authors = [{ name = "Hynek Schlawack", email = "hs@ox.cx" }]
-# Keep in sync with noxfile.py/tests_oldest_attrs
 requires-python = ">=3.7"
 license = "Apache-2.0"
 keywords = ["app", "config", "env", "cfg"]
@@ -31,11 +30,7 @@ classifiers = [
     "Programming Language :: Python :: 3.12",
     "Topic :: Software Development :: Libraries :: Python Modules",
 ]
-dependencies = [
-    # Keep in sync with noxfile.py/tests_oldest_attrs
-    "attrs>=17.4.0",
-    "importlib_metadata; python_version<'3.8'",
-]
+dependencies = ["attrs>=17.4.0", "importlib_metadata; python_version<'3.8'"]
 
 [project.optional-dependencies]
 tests = ["pytest", "coverage[toml]", "moto"]