Faster Tests (DjangoCon)

[antonpirker]

This fixes #1499

• Trying the sentry-ruby way (splitting up into multiple yaml files, this works great. I have created a script to split tox.ini into multiple yaml files automatically)
• Trying tox-gh (does not help us, because we already have this paralleization in GitHub actions)
• Cleaning up the yaml file in general.
• Using a RAM drive for the tox virtualenvs in the GitHub runners
• Removed PyPy from the test suite because it was never run. We have to reevaluate support for PyPy.

[sl0thentr0py]

ok firstly, great work!
just some random comments and one more here.

Can you document in the PR description / contributing guide what we need to do if we add/remove a certain tox configuration in tox.ini? (basically just run the script again, I guess.)

[sl0thentr0py]

[antonpirker]

I have now made it so the CI fails when tox.ini and the existing yaml files are out of sync: https://github.com/getsentry/sentry-python/actions/runs/3082611176/jobs/4982524192