Use nox to further automate our release mechanisms

[pradyunsg]

Not sure why I kept working on this past 2am. But hey, now we have our release process reduced significantly.

• Get rid of generate_* nox commands, since, well, there's no point in invoking them directly if everything is automated.
• Create a new tools/automation/release/__init__.py file, that contains helpers for release process automation.
• Add a new build-release command, to go from tagged release -> distributions in dist/.
• Nicer version bumping scheme that's in-compliance with our current release cadence.

Other than that, the fact that 7150861 needed commits to be made to the branch, meant that I was completely confused while working on it and lost my work once. Anyway, I think this works, since I managed to get to a distribution using this two commands on my machine. :P

[xavfernandez]

It might be the occasion to clear up the workflow due to the fact that we cannot push directly on master.
What I did for 19.3 was:

• creating a pull request after "Prepare for release using nox -s prepare-release -- YY.N" (without pushing the tag)
• wait for the CI and merge the PR with a merge commit no squash or rebase to keep the tagged commit
• only after the PR was merged did I push the matching tag and uploaded the new version to PyPI.

[pradyunsg]

It might be the occasion to clear up the workflow due to the fact that we cannot push directly on master.

Let's address that in a follow up -- I do nearly the same thing.

The only difference is that I disable the branch protections (so that I can merge the PR, before CI passes, by checking a checkbox to say "YES I KNOW WHAT I'M DOING") and then re-enable protections in the same minute.

The bugfix process also needs clarification, so there's that.

[pradyunsg]

I'll merge this since I don't think the changes since @xavfernandez's approval are significant changes (mostly just a CI update I'd missed and addressing his comment).