Reduce redundant information in _process_plot_var_args.

[anntzer]

_prop_keys is redundant with _cycler_items and not really needed anywhere. Removing it makes it less likely that a _process_plot_var_args gets in an invalid state if _cycler_items is updated while _prop_keys is not touched.

Inspired by #26831.

A second separate commit adds some minor cleanups.

PR summary

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• 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

[tacaswell]

failures look unrelated (build system looks like it is fighting back):

ERROR  -   File "/home/runner/work/matplotlib/matplotlib/lib/matplotlib/_version.py", line 4
131
    __version__ : str = version : str = '3.9.0.dev259+gf912eb0767'
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                                ^
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SyntaxError: invalid syntax
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ERROR  -   File "/home/runner/work/matplotlib/matplotlib/lib/matplotlib/_version.py", line 4
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    __version__ : str = version : str = '3.9.0.dev259+gf912eb0767'
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                                ^
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SyntaxError: invalid syntax

[ksunden]

Yes, that resulted from the (now yanked) setuptools-scm v8.0.0, so restarting tests now that that shouldn't be a problem.

[ksunden]

Remaining CI failures appear to be #26846 (plus the doc build which seems to have not rebooted on that power cycle, but this is only touching private functionality anyway, so not too concerned about that)