bpo-40882: Fix a memory leak in SharedMemory on Windows (GH-20684)

In multiprocessing.shared_memory.SharedMemory(), the temporary view
returned by MapViewOfFile() should be unmapped when it is no longer
needed.
(cherry picked from commit 85c128e34daec7625b74746e127afa25888ccde1)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>