test_drain_raises (test.test_asyncio.test_streams.StreamTests) ... Exception in thread Thread-20:
Traceback (most recent call last):
  File "/home/greg/oss/cpython/Lib/threading.py", line 938, in _bootstrap_inner
    self.run()
  File "/home/greg/oss/cpython/Lib/threading.py", line 876, in run
    self._target(*self._args, **self._kwargs)
  File "/home/greg/oss/cpython/Lib/test/test_asyncio/test_streams.py", line 951, in server
    with socket.create_server(('localhost', 0)) as sock:
  File "/home/greg/oss/cpython/Lib/socket.py", line 804, in create_server
    raise error(err.errno, msg) from None
OSError: [Errno 99] Cannot assign requested address (while attempting to bind on address ('localhost', 0))

I'm preparing an IPv6-only buildbot.  (there is no IPv4 localhost)