[3.12] gh-88834: Unify the instance check for typing.Union and types.UnionType (GH-128363) (GH-128371)

Union now uses the instance checks against its parameters instead of
the subclass checks.
(cherry picked from commit b2ac70a62ad1be8e037ce45ccf5f1b753ea5e64b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>