Asyncio-dgram

- Add license to setup.py [Fabian Affolter]

- AF_UNIX support added for python 3.7+

- bind() no longer explicitly disables SO_REUSEADDR as upstream deprecated
the option.

- bind() no longer sets SO_REUSEADDR by default. See upstream discussion
for rationale, https://bugs.python.org/issue37228

- DatagramStream.send() will now wait for the asyncio write buffer to
drain below the highwater mark before returning. This mimics the
forthcoming behavior from upstream, https://bugs.python.org/issue38242