gh-155003: Don't try to close mmap in SharedMemory.__del__

[takluyver]

This is an alternative to #155007, keeping the file descriptor in the object and fixing the __del__ method to close it without trying to close the mmap or memoryview, which was my original idea in #155003.

I could add, in this PR or separately:

• The same posix_fallocate() change as in gh-155003: Don't keep file descriptor in multiprocessing SharedMemory object #155007 - but with this option it's orthogonal.
• A public API to access the fd, probably a .fileno() method, which is a convention for higher level objects which wrap an fd.
• Use mmap(..., trackfd=False) to skip the duplicate fd within mmap (at a cost of breaking potential workarounds using sm._mmap.resize() & .size())

• Issue: multiprocessing SharedMemory.__del__ can fail, leak file descriptor #155003