Additionally, allow the caller to disable the default listener on 127.0.0.1.

By detecting the address family rather than assuming AF_INET,
we allow unix domain sockets to be inherited properly.

Setuptools handled this fine but tox skips installing modules within
the top-level package

Eliminate dependency on httpunixsocketconnection; it does not declare
compatibility with Python 3.7.

Patching an HTTPConnection object like this is hackish, but may be good
enough for a test suite. I think it's pretty unlikely to lead to a false
negative.