bpo-34172: multiprocessing.Pool leaks resources after being deleted (GH-8450) (GH-9676)

Fix a reference issue inside multiprocessing.Pool that caused the pool to remain alive if it was deleted without being closed or terminated explicitly.
(cherry picked from commit 97bfe8d3ebb0a54c8798f57555cb4152f9b2e1d0)

Co-authored-by: tzickel <tzickel@users.noreply.github.com>
3 files changed