Make colorbar boundaries work again

[jklymak]

PR Summary

colorbar has a boundaries kwarg, which is supposed to make the colorbar boundaries hard coded. This was broken by the colorbar rewrite.

I actually think this kwarg is a bit of an abomination from a data visualization point of view; if you want discrete colors that is the job of the norm, not the colorbar. But I also admit we can't break it.

Added a new test so we don't break it wantonly again.

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout v3.5.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -m1 5cd6d69da4db8960c7d2475224a377ce4418efc0

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #21913: Make colorbar boundaries work again'

4. Push to a named branch:

git push YOURFORK v3.5.x:auto-backport-of-pr-21913-on-v3.5.x

5. Create a PR against branch v3.5.x, I would have named this PR:

"Backport PR #21913 on branch v3.5.x (Make colorbar boundaries work again)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.

[QuLogic]

This conflicts with #21137 (on main but not in v3.5.x), and I'm not sure exactly how to fix it, so I'm going to bump this to 3.5.2.

[QuLogic]

Actually, I just looked closer at what that did and I'm just going to backport both at the same time.