Tests don't run on full matrix

[chrysn]

With the setup.py tests breaking down and the migration to tox, not all scenarios can be easily tested any more. Possibly due to https://bugs.debian.org/962654 (where the workaround is some help but doesn't solve everything), testing pypy in combination with uvloop currently does not work and is removed from the test suite.

Help in getting all variations covered (py37, py38, pypy against various loops and sets of installed extras) would be appreciated, and is expected to require good understanding of which the components involved in venv setups.