MEP30: make each "pseudotype" a class - #17446

In the specific case of capstyle and joinstyle, I think it should be relatively easy* to turn them into StrEnums (like IntEnums, but for strs...), i.e.

class CapStyle(str, Enum):
    butt = "butt"
    round = "round"
    projecting = "projecting"

and put the doc there together with whatever relevant methods, and just call it a day.

(*for mpl-specific values of "easy", conditions may apply)

A discussed on the call, I like this ideas in terms of documentation/writing a matplotlib user manual of sorts...

I'm way more hesitant about it as code w/out a clear articulation of which parameters should be pseudotypes. But as a first pass, I think this intersects w/ #6738 since doing that properly would require a validation method per parameter.

xref #14642 on enums.

Also, s/matplotlib/Matplotlib/g, mostly.

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I propse to close this as too heavy. I think the solution is somewhere between StrEnums and Literal type annotations, c.f. #14642.

Since this Pull Request has not been updated in 60 days, it has been marked "inactive." This does not mean that it will be closed, though it may be moved to a "Draft" state. This helps maintainers prioritize their reviewing efforts. You can pick the PR back up anytime - please ping us if you need a review or guidance to move the PR forward! If you do not plan on continuing the work, please let us know so that we can either find someone to take the PR over, or close it.