GH-142928: Add deprecation warning for removal of color on HelpFormatter

[savannahostrowski]

Never documented and buggy but alas, still used, so we need a deprecation warning. More context here: #142274 (comment)

• Issue: argparse.HelpFormatter color argument removed in 3.15.0a3 without deprecation #142928

[hroncok]

Thanks. Should this still attempt to call self._set_color(color) in this case? Or else if it cannot, say "deprecated and does nothing" in the warning/docs?

[hugovk]

It's undocumented, fix the Sphinx warning:

Suggested change

	* The *color* parameter of :class:`~argparse.HelpFormatter` is deprecated.
	* The *color* parameter of :class:`!HelpFormatter` is deprecated.

[hugovk]

Suggested change

	Deprecate :class:`~argparse.HelpFormatter`'s *color* parameter. Users should use the *color* parameter on :class:`~argparse.ArgumentParser` instead.
	Deprecate :class:`!HelpFormatter`'s *color* parameter. Use the *color* parameter on :class:`~argparse.ArgumentParser` instead.