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

tests/run-tests.py

@@ -260,7 +260,9 b' def checkportisavailable(port):'
             s.bind(('localhost', port))
         return True
     except socket.error as exc:
-        if exc.errno not in (
+        if os.name == 'nt' and exc.errno == errno.WSAEACCES:
+            return False
+        elif exc.errno not in (
             errno.EADDRINUSE,
             errno.EADDRNOTAVAIL,
             errno.EPROTONOSUPPORT,