Setup black, isort and pytest

[KapJI]

• Run them using pre-commit.
• Setup CI job on Github Actions which runs all pre-commit checks.
• Remove tox config. Poetry already provides venv and different python versions are more easily tested on Github Actions.

Another part of #26.

Closes #7

[KapJI]

I checked multiple times that path to CI config is correct, I think it will run only after merging this.

[simon-weber]

looks good! I'll set up Actions and try to get it running on a copy of this branch.

[KapJI]

Not sure, but probably it will only work after being committed to the default branch (master).

[simon-weber]

Looks good. Here's a test run with no changes: https://github.com/simon-weber/gpsoauth/runs/2247269621

And here's one with some failures from --no-verify: https://github.com/simon-weber/gpsoauth/runs/2247329653

[simon-weber]

And here's the first run on master! https://github.com/simon-weber/gpsoauth/actions/runs/709002120 🎉