[3.13] gh-132713: Fix typing.Union[index] race condition (GH-132802) (#132839)

gh-132713: Fix typing.Union[index] race condition (GH-132802)

Add union_init_parameters() helper function. Use a critical section
to initialize the 'parameters' member.
(cherry picked from commit dc3e9638c22fc1fa807a88c32316ac2558a4b879)

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