fix: use trio.lowlevel instead of trio.hazmat with Trio >= 0.15

[ntamas]

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)

Description:

This PR gets rid of two deprecation warnings that appear when TrioEventLoop is used in conjunction with trio >= 0.15. Trio 0.15 deprecated the trio.hazmat module in favour of trio.lowlevel.

[coveralls]

Coverage decreased (-0.04%) to 77.833% when pulling a57465c on collmot:fix/trio-hazmat-deprecation-warning into 4c739b6 on urwid:master.

[ntamas]

Note that trio 0.18 has now removed the deprecated trio.hazmat module so TrioEventLoop will not work with Trio >= 0.18 until this PR is merged.

[pquentin]

This pull request is fine, but it's probably simpler to just use trio.lowlevel, trio 0.15 is now pretty old by trio standards.

[tonycpsu]

Thanks, and sorry for the delay in merging.