Deprecate MarkerStyle(None).

[anntzer]

See docstring change for the confusion that this resolves.
We can normalize None inputs on the caller side (and note that directly
constructing MarkerStyles is not something that most users need to do).

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[dstansby]

I tried but struggled to work out what's going on here with the change?

[anntzer]

The test simply draws all markers in a single figure, ordered by str(type(x))+str(x) (not all x have same type: markers can be str (most of them), but also int or, previously, None, hence the need to convert to str). A previous PR introduced MarkerStyle("none"), i.e. "none" was added into the MarkerStyle.markers dict; in order to not change the baseline image, I had to remove it from the list of entries. With this PR, I additionally removed None from the dict; again, in order to not change the baseline image, I had to put it back (using its synonym "none") into the list of entries (at the position where it was previously, i.e. in from of everything else).

[timhoffm]

I would write this as

    markers = sorted(matplotlib.markers.MarkerStyle.markers,
                     key=lambda x: str(type(x))+str(x))
    # Since generation of the test image, None was removed but 'none' was
    # added. By moving 'none' to the front (=former sorted place of None)
    # we can avoid regenerating the test image. This can be removed if the
    # test image has to be regenerated for other reasons.
    markers.remove('none')
    markers = ['none', *markers]

    for y, marker in enumerate(markers):

[anntzer]

sure, done