CI: Pin matplotlib to < 3.7.0

[phofl]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[jbrockmendel]

whats the condition under which we can revert the pin?

[phofl]

#51394 links a failing build.

Not really familiar with how matplotlib works, but couldn't find a whatsnew that references this

[MarcoGorelli]

whats the condition under which we can revert the pin?

taking plotting out of pandas 😄

[MarcoGorelli]

There's failures because of both MPL and openpyxl, and here you're only pinning MPL (hence some CI failures)?

Looks good to me

[phofl]

Yep, merging both should get ci green.

[phofl]

Makes reverting easier when doing it separately

[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 1.5.x
git pull

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

git cherry-pick -x -m1 dd30415b6e569b66051d41172f86cce79244f519

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

git commit -am 'Backport PR #51393: CI: Pin matplotlib to < 3.7.0'

4. Push to a named branch:

git push YOURFORK 1.5.x:auto-backport-of-pr-51393-on-1.5.x

5. Create a PR against branch 1.5.x, I would have named this PR:

"Backport PR #51393 on branch 1.5.x (CI: Pin matplotlib to < 3.7.0)"

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.