Fix reshape list of strings

[jklymak]

PR Summary

Closes #18234

cbook._reshape_2D was altered to check for iterables in a list, which is great, except categorical strings are iterables. This broke:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
n, bins, patches = ax.hist(['a', 'b', 'b', 'c', 'd', 'ff', 'ff', 'a'])
plt.show()

Before

After

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/next_api_changes/* if API changed in a backward-incompatible way

[jklymak]

I hate to hold up 3.3.1 for this, but its a pretty bad regression. Feel free to re-milestone if you don't agree...

[jklymak]

BTW, I guess we can argue about the usefulness of categoricals with hist, but...

[tacaswell]

I'm investigating the OSX failures, they seem to be transient...

[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 v3.3.x
$ git pull

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

$ git cherry-pick -m1 17a0ff73282c1d8699abd29670b2d42505393c45

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

$ git commit -am 'Backport PR #18243: Fix reshape list of strings'

4. Push to a named branch :

git push YOURFORK v3.3.x:auto-backport-of-pr-18243-on-v3.3.x

5. Create a PR against branch v3.3.x, I would have named this PR:

"Backport PR #18243 on branch v3.3.x"

And apply the correct labels and milestones.

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

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

[QuLogic]

I'll add this to #18247.