Add Python 3.9 to tox run.

[carltongibson]

Follow up from #78.

I had to drop PY39 as it wasn't installing properly in tox.

envlist =
    flake8-py39,
    py{36,37,38}-dj{22,31,32},
    py{38}-dj{main}

Just need to add the ,39s in the two py{...} declarations there to run it.

See https://github.com/django-compressor/django-appconf/runs/3394353179?check_suite_focus=true#step:5:43 for log.

[jerinpetergeorge]

it seems like an issue with the coverage command (same as nedbat/coveragepy#862 ? 🤔 that fixed in v5.0.3). I was able to run the tests without coverage command