Update handling of sequence labels for plot

[rcomer]

PR summary

Closes #27762.

• plot(x, y, label=['foo']) where y is 1D now results in the legend label "foo" instead of "['foo']" (immediate change).
• plot(x, y, label=sequence) where y is 1D and sequence is not of length 1 is deprecated to error in future.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[timhoffm]

@rcomer I've introduced #27780, which should resolve the boxplot issues you've had with this PR. I suggest to wait until that gets merged, then rebase on it and remove all the boxplot specifics here.

[rcomer]

Hid a bunch of comments because they discuss issues with boxplot that are now being handled elsewhere. This change no longer touches boxplot.

[timhoffm]

Optional but inverting the if branches is more readable:

Suggested change

	if len(label) != n_datasets:
	if len(label) == n_datasets:
	…

• you remove the negation in the condition
• And the short and good-case branch comes first

[rcomer]

I'm a little sleepy but I think this if doesn't have an else.

[rcomer]

Oh, I think I get it. I can have a look this evening (on the wrong computer at the moment).

[ksunden]

Anyone can merge on green CI