run-tests: catch a Windows specific error when testing for a free socket
I'm not sure why this only happens with py3, but this error code doesn't map to
any of the 3 currently being handled, and kills `run-tests.py` before it can run
any tests when it happens:
OSError: [WinError 10013] An attempt was made to access a socket in a way
forbidden by its access permissions
The documentation[1] says this can happen if another process is bound to the
address with exclusive access. This seems to keep it happy.
[1] https://docs.microsoft.com/en-us/windows/win32/winsock/windows-sockets-error-codes-2
Differential Revision: https://phab.mercurial-scm.org/D9816
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