bpo-40521: Empty frozenset is no longer a singleton (GH-21085)

* Revert "bpo-40521: Make the empty frozenset per interpreter (GH-21068)"

This reverts commit 261cfedf7657a515e04428bba58eba2a9bb88208.

* bpo-40521: Empty frozensets are no longer singletons

* Complete the removal of the frozenset singleton