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 says this can happen if another process is bound to the
address with exclusive access. This seems to keep it happy.