https://github.com/ambv/black

So that failing tests don't cause the server thread to hang
indefinitely, waiting for connections that will never come.

Rationale for suppressing error/traceback from interrupted
_accept_connection in testserver.Server:
https://gist.github.com/brettdh/b6e741227b2297f19d2118077f14dfa5

* Move server socket close to just before join

This way it handles the no-connections, no-exceptions case
as well as the exception case. If the server thread doesn't
exit by itself within 5 seconds of the context manager exit,
the accept will be interrupted.

* Address feedback

- pytest.raises rather than except:pass
- Move socket create/bind back to run()
- Timeout on accepting connections

It seems it's necessary both in pulling all_proxy from the environment
(rebuild_proxies) and deciding which proxy to use (select_proxy).

Also added new functional test.