gh-91577: SharedMemory move imports out of methods (GH-91579)

SharedMemory.unlink() uses the unregister() function from resource_tracker. Previously it was imported in the method, but this can fail if the method is called during interpreter shutdown, for example when unlink is part of a __del__() method.

Moving the import to the top of the file, means that the unregister() method is available during interpreter shutdown.

The register call in SharedMemory.__init__() can also use this imported resource_tracker.
(cherry picked from commit 9a458befdd68625d088f4fea7df135a57d147deb)

Co-authored-by: samtygier <samtygier@yahoo.co.uk>