Fix pytest warnings, move event_loop to conftest.py

[andrew-fleming]

This PR proposes to add auto mode to pytest-asyncio in tox.ini. This eliminates the myriad deprecation warnings from pytest when running tests (while also keeping the code up to date). Note that users can override pytest-asyncio with other async frameworks if they prefer.

This PR also proposes to add the event_loop to the conftest.py module. In doing so, all other event_loop instances can be discarded.