bpo-31151: Add socketserver.ForkingMixIn.server_close() (#3057)

* Add socketserver.ForkingMixIn.server_close()

bpo-31151: socketserver.ForkingMixIn.server_close() now waits until
all child processes completed to prevent leaking zombie processes.

* Fix test on Windows which doesn't have ForkingMixIn
3 files changed