fix: remove ConnectorLoopError from connector.connect

[jackwotherspoon]

Python packages like gevent have monkey patching abilities and use greenlet
processes to replace synchronous code in an "asynchronous" fashion.

The ConnectorLoopError in connector.connect was put in as a safe-guard to try
and prevent improper async driver usage where users would accidentally call
connector.connect for asyncpg instead of calling connector.connect_async,
leading to an indefinite hang.

However, this error is preventing support for gevent which schedules tasks in
its greenlet threads.

Closes #969

[jackwotherspoon]

Originally added an integration test, but have since removed it as there seems to be no clean way to un-monkeypatch the modules afterwards... I have manually tested both gevent and eventlet now work.

[enocom]

Looks great! I confirmed this code works as expected when using gevent.