Two test_multiprocessing_spawn.test_misc tests are still failing and requires more investigation:

• FAIL: test_resource_tracker_sigterm (test.test_multiprocessing_spawn.test_misc.TestResourceTracker.test_resource_tracker_sigterm)
  test.test_multiprocessing_spawn.test_processes
• FAIL: test_wait_result (test.test_multiprocessing_spawn.test_processes.WithProcessesTestCondition.test_wait_result)

The forkserver start method requires multiprocessing.reduction.HAVE_SEND_HANDLE which is False on Cygwin:

HAVE_SEND_HANDLE = (sys.platform == 'win32' or
                    (hasattr(socket, 'CMSG_LEN') and
                     hasattr(socket, 'SCM_RIGHTS') and
                     hasattr(socket.socket, 'sendmsg')))

I disabled SCM_RIGHTS and sendmsg() in the socket module on Cygwn.

The "win32" implementation uses the _winapi extension which is not available on Cygwin.

Disable fork and forkserver start methods on Cygwin

I tested again and in fact, it's possible to support the fork start method on Cygwin. I close this PR and I wrote PR gh-150031 instead.