Run type checks only against packaged *.py files

- for tests/ and the noxfile.py, type annotations are not strictly
  enforced any more
  + this simplifies the way test cases and nox sessions are written
  + for many pytest fixtures, no types are available via a public API
- put fixtures inside the classes the corresponding test cases are
  grouped in

setup.cfg

@@ -96,6 +96,8 @@ extend-ignore =
 
 per-file-ignores =
     noxfile.py:
+        # Type annotations are not strictly enforced.
+        ANN0, ANN2,
         # TODO (isort): Check if still too many module members.
         WPS202,
         # TODO (isort): Remove after simplifying the nox session "lint".
@@ -103,6 +105,8 @@ per-file-ignores =
         # No overuse of string constants (e.g., '--version').
         WPS226,
     tests/*.py:
+        # Type annotations are not strictly enforced.
+        ANN0, ANN2,
         # `assert` statements are ok in the test suite.
         S101,
         # Shadowing outer scopes occurs naturally with mocks.
@@ -177,7 +181,7 @@ single_line_exclusions = typing
 [mypy]
 cache_dir = .cache/mypy
 
-[mypy-nox.*,packaging.*,pytest,_pytest.*]
+[mypy-nox.*,packaging,pytest]
 ignore_missing_imports = true