BUG (string dtype): correctly enable idxmin/max for python-storage strings

[jorisvandenbossche]

The argmin/argmax methods already work fine on the EAs, but it was not yet properly wired up in EA._reduce for the python storage variant.

xref #54792

[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 2.3.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 754d09163ae08f2b87daa41f2263556dbb809616

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

git commit -am 'Backport PR #60242: BUG (string dtype): correctly enable idxmin/max for python-storage strings'

4. Push to a named branch:

git push YOURFORK 2.3.x:auto-backport-of-pr-60242-on-2.3.x

5. Create a PR against branch 2.3.x, I would have named this PR:

"Backport PR #60242 on branch 2.3.x (BUG (string dtype): correctly enable idxmin/max for python-storage strings)"

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.

[jorisvandenbossche]

Manual backport -> #60256