bpo-44184: Fix subtype_dealloc() for freed type (GH-26274) (GH-26290)

Fix a crash at Python exit when a deallocator function removes the
last strong reference to a heap type.

Don't read type memory after calling basedealloc() since
basedealloc() can deallocate the type and free its memory.

_PyMem_IsPtrFreed() argument is now constant.
(cherry picked from commit 615069eb08494d089bf24e43547fbc482ed699b8)

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

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