Python 3.10 support

[stefanor]

Python 3.10 dropped support for the loop parameter to asyncio.sleep() and gather(), the default loop is used instead. I haven't audited whether any of these code paths now need to use set_default_loop(), but tests pass, so... LGTM?

Support for using the default loop was only added in 3.7, so this drops support for Python 3.6.

https://bugs.python.org/issue42392

Fixes: #137