Add support for Unix domain sockets

[vkottler]

• https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.create_unix_server
• https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.create_unix_connection
• https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.create_datagram_endpoint

add support for all 3 of these interfaces. unit test can just be marked as only running on Linux

[vkottler]

start with simple unit test server + client socket based on temp filename

[vkottler]

need to also test if Python can do both SOCK_STREAM and SOCK_DGRAM types, what about SOCK_SEQPACKET?