PR Summary

Closes #20258

Passing 'face' to a polygon that would show up in the legend would not be recognized. Now it is...

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.fill_between([0, 1, 2], [1, 2, 3], [2, 3, 4], 
                facecolor='r', edgecolor='face', label='Fill')
ax.legend()
plt.show()

before:

ValueError: Using a string of single character colors as a color sequence is not supported. The colors can be passed as an explicit list instead.

Now:

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).