PR Summary

As reported on gitter, if an axes does not have a subplotspec align_ylabels will fail:

fig, ax = plt.subplot_mosaic('AABB', figsize=(5, 2), constrained_layout=True)
pm = ax["A"].pcolormesh(np.random.randn(100, 100))
plt.colorbar(pm, ax=ax['A'], shrink=0.69, pad=0)
fig.align_ylabels()

This PR fixes this and tests the fix.

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• 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).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).