PR Summary

Closes #20760

If you specify the x-position of an supxlabel (or y position of supylabel) it should not disqualify the label from constrained_layout...

import matplotlib.pyplot as plt

fig, ax = plt.subplots(constrained_layout=True)
lab = fig.supxlabel('Boo', x=0.7)
plt.show()

Before:

After

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).