Remove more Python 3.8 cruft (#13787)

tests/mypy_test.py

@@ -393,8 +393,7 @@ def stdlib_module_name_from_path(path: Path) -> str:
     assert path.suffix == ".pyi"
     parts = list(path.parts[1:-1])
     if path.parts[-1] != "__init__.pyi":
-        # TODO: Python 3.9+: Use removesuffix.
-        parts.append(path.parts[-1][:-4])
+        parts.append(path.parts[-1].removesuffix(".pyi"))
     return ".".join(parts)