CI: Unpin matplotlib
- [ ] closes #51394 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) 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.

lets see if the new release addresses the failures
cc @mroeschke I think you took a closer look at the failures when 3.7 came out. Is this test failure expected and we have to fix it or is it on the matplotlib side? I don't remember seeing `UserWarning: All values for SymLogScale are below linthresh, making it effectively linear. You likely should lower the value of linthresh.` in the previous release. I suppose we can catch this in our test.

For `AttributeError: 'NoneType' object has no attribute 'rowspan'` I think we have to address this on our side. I tried addressing it with this (undo) commit but it caused other test failures I didn't really understand :/
https://github.com/pandas-dev/pandas/pull/51470/commits/a36f00790a5468186869b8e0dc186cc6189bb672 #52150 fixes one of the errors.

The other failure I think you should fix by making the data range bigger with your test data or add a warning filter to that test. 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 2.0.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 2c68dd3d9c415f40b04c500936670470c4eabd6e
```

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

```
git commit -am 'Backport PR #51990: CI: Unpin matplotlib'
```

4. Push to a named branch:

```
git push YOURFORK 2.0.x:auto-backport-of-pr-51990-on-2.0.x
```

5. Create a PR against branch 2.0.x, I would have named this PR:

> "Backport PR #51990 on branch 2.0.x (CI: Unpin matplotlib)"

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](https://github.com/MeeseeksBox/MeeseeksDev).
                