Pin major version of all dependencies

Under Semantic Versioning (common for Python packages), breaking API
changes can occur in a major version update. This has happened with
isort 5 and could happen with any other package that flake8-isort or
its Travis build uses.

.travis.yml

@@ -15,6 +15,6 @@ script:
   - flake8 *.py
   - pytest run_tests.py
 after_success:
-  - pip install -q coverage==4.4 coveralls pytest-cov
+  - pip install -q 'coverage==4.4' 'coveralls<2' 'pytest-cov<3'
   - pytest run_tests.py --cov flake8_isort --cov-report term-missing
   - coveralls

CHANGES.rst

@@ -6,7 +6,10 @@ Changelog
 3.0.1 (unreleased)
 ------------------
 
-- Nothing changed yet.
+- Work around `FailedToLoadPlugin` exception by requiring `isort` 4.x. Likewise,
+  pin the major version of all dependencies, to reduce risk of any future
+  incompatibilities.
+  [pkolbus]
 
 
 3.0.0 (2020-04-15)

setup.py

@@ -56,13 +56,13 @@ def get_version(file="flake8_isort.py"):
     test_suite='run_tests',
     zip_safe=False,
     install_requires=[
-        'flake8 >= 3.2.1',
-        'isort[pyproject] >= 4.3.5',
-        'testfixtures',
+        'flake8 >= 3.2.1, <4',
+        'isort[pyproject] >= 4.3.5, <5',
+        'testfixtures >= 6.8.0, <7',
     ],
     extras_require={
         'test': [
-            'pytest',
+            'pytest >= 4.0.2, <6',
         ],
     },
     entry_points={