[BREAKING CHANGE] Refactoring: Split event loop in several modules

[penguinolog]

• urwid.main_loop is split into multiple modules which is easier to maintain
• urwid.compat is not used anymore and removed
• TornadoEventLoop, GLibEventLoop, TwistedEventLoop and TrioEventLoop accessible ONLY if required dependencies installed (like: Tornado installed -> TornadoEventLoop is accessible for import)
• TornadoEventLoop use the same idle logic as AsyncioLoop: tornado.ioloop.IOLoop is asyncio based.
• Trio < 0.15 is not supported. Version 0.15 was released almost 3 years ago.
• Tornado < 5.0 is not supported. Tornado 5.0 was released 5 years ago.

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

[wardi]

What's the breaking change part of this? It looks like the __init__.py and conditional imports will remain compatible

[penguinolog]

What's the breaking change part of this? It looks like the __init__.py and conditional imports will remain compatible

Direct import of event loop class from implementation module changed (since module splitted).
Stubs import impossible without requirements met (should not be used, but can expect anything)
self.GLib like hacks removed, probably somebody used it (weird)

[wardi]

LGTM