Add optional requires_python field to third-party stubs metadata (#10724)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

tests/utils.py

@@ -9,7 +9,7 @@ import sys
 import venv
 from functools import lru_cache
 from pathlib import Path
-from typing import Any, NamedTuple
+from typing import Any, Final, NamedTuple
 from typing_extensions import Annotated
 
 import pathspec
@@ -22,6 +22,9 @@ except ImportError:
         return text
 
 
+PYTHON_VERSION: Final = f"{sys.version_info.major}.{sys.version_info.minor}"
+
+
 # A backport of functools.cache for Python <3.9
 # This module is imported by mypy_test.py, which needs to run on 3.8 in CI
 cache = lru_cache(None)