[3.14] gh-146444: Make Platforms/Apple/ compatible with Python 3.9 (GH-146624) (#146627)

gh-146444: Make Platforms/Apple/ compatible with Python 3.9 (GH-146624)

Replace "str | None" with typing.Union[str, None].
(cherry picked from commit 382c04308d7c3638fc0402116ce8654b80b4b776)

Co-authored-by: Victor Stinner <vstinner@python.org>