Use multiprocessing.shared_memory instead of posixshmem

Since Python 3.8, the stdlib has multiprocessing.shared_memory which does basically the same as posixshmem. It's not better than posixshmem, but it's one less dependency to pull in. There are some small differences in the API; the multiprocessing one thinks more in terms of whether files exist (and whether to create them) rather than whether to rw; see the small diff for some exception handling. I plan to add some more tests.