Automate populating version targets in tox

[sentrivana]

Right now, we add new versions of integrations to test against to tox.ini whenever someone notices there has been a release (not very often).

Write a script that instead does this automatically:

• parse tox.ini to figure out the lowest supported version of each integration (i.e., the lowest version we test with)
• fetch package releases from PYPI
• find out what the latest release of the integration is
• pick out a handful of releases in between oldest supported and latest
• figure out which Python versions they should be tested on
• generate tox.ini with the new targets
• regenerate the workflow yaml files as well (scripts/split-tox-gh-actions/split-tox-gh-actions.py)