Enable Ruff D (pydocstyle) with pep257 convention (#13326)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/mypy_test.py

@@ -62,7 +62,7 @@ class CommandLineArgs:
 
 
 def valid_path(cmd_arg: str) -> Path:
-    """Helper function for argument-parsing"""
+    """Parse a CLI argument that is intended to point to a valid typeshed path."""
     path = Path(cmd_arg)
     if not path.exists():
         raise argparse.ArgumentTypeError(f'"{path}" does not exist in typeshed!')
@@ -72,7 +72,10 @@ def valid_path(cmd_arg: str) -> Path:
 
 
 def remove_dev_suffix(version: str) -> str:
-    """Helper function for argument-parsing"""
+    """Remove the `-dev` suffix from a version string.
+
+    This is a helper function for argument-parsing.
+    """
     if version.endswith("-dev"):
         return version[: -len("-dev")]
     return version
@@ -303,7 +306,6 @@ def test_third_party_distribution(
     Return a tuple, where the first element indicates mypy's return code
     and the second element is the number of checked files.
     """
-
     files: list[Path] = []
     configurations: list[MypyDistConf] = []
     seen_dists: set[str] = set()