bpo-45310: Fix parrallel shared memory tests (GH-28661)

Add a PID to names of POSIX shared memory objects to allow
running multiprocessing tests (test_multiprocessing_fork,
test_multiprocessing_spawn, etc) in parallel.
(cherry picked from commit eb4495e8)

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