Enable all Python warnings in the test suite

[kurtmckee]

This PR enables all warnings in the test suite when run in tox.

With this change, additional ResourceWarning warnings display. For example:

sys:1: ResourceWarning: unclosed file <_io.BufferedRandom name='/tmp/dramatiq-prometheus/gauge_all_2653069.db'>
sys:1: ResourceWarning: unclosed file <_io.BufferedRandom name='/tmp/dramatiq-prometheus/gauge_livesum_2653069.db'>
sys:1: ResourceWarning: unclosed file <_io.BufferedRandom name='/tmp/dramatiq-prometheus/histogram_2653069.db'>
sys:1: ResourceWarning: unclosed file <_io.BufferedRandom name='/tmp/dramatiq-prometheus/counter_2653069.db'>

Through trial and error I was able to isolate these four open files to the following test (other tests in that file only trigger warnings about three or fewer unclosed files):

tox -e py38 -- tests/test_actors.py::test_actors_can_delay_messages_independent_of_each_other

I've not yet identified how these files are getting opened, but with the changes in this PR, it'll be obvious when this issue is resolved.