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This test is inherently subject to a race condition:
"""
 port = support.find_unused_port()
 [...]
 try:
 self.assertRaises(OverflowError, sock.bind, (host, big_port))
 self.assertRaises(OverflowError, sock.bind, (host, neg_port))
 sock.bind((host, port))
 finally:
 sock.close()
"""
The buildbot being set up ton run the test suite with "-j4" is
probably the most important factor.
A possibility would be to try the find_unused_port() + bind() in a
loop a couple times.