Support vmin/vmax with bins='log' in hexbin

[svank]

PR Summary

Matplotlib 3.3 deprecated passing norm and vmin or vmax parameters simultaneously to certain functions. The hexbin plotting method supports a bins='log' parameter that applies logarithmic scaling to the colormap of the resulting histogram. Under the hood, this behavior is implemented by generating a LogNorm and setting it to the norm parameter. This means that plt.hexbin([], [], bins='log', vmin=1) raises a deprecation warning (or error in the dev branch), Passing parameters norm and vmin/vmax simultaneously is deprecated... Please pass vmin/vmax directly to the norm when creating it.

This error is confusing since the user didn't create a norm, and the bins='log' parameter seems to be a convenience to let the user avoid importing and creating a LogNorm, so I think any vmin/vmax parameters should be accounted for when creating the LogNorm under the hood. This PR does that and adds a test for this situation.

The incompatibility between bins='log' and vmin/vmax is not mentioned in the docs, so I don't think this PR needs any doc changes.

PR Checklist

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