Previously, moving the cursor over
plt.imshow(1 + np.random.randn(10, 10) * 1e-15) would result in
ValueError: math domain error (when trying to compute
math.log10(delta) with delta = 0). This is because the full range
of normalization implied by the image above (~1e-15) is representable
with float precision, but the spacing between individual colors (256x
less) is too small to be represented, and thus rounded to zero.

In general, I consider that such floating point errors should be fixed
on the user side, but this specific error seems worth fixing as the 1)
the image is still rendered correctly, 2) errors in the mouse event
handler (rendering the cursor text) are somewhat obscure for end users,
and 3) it used to work before Matplotlib 3.5.

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).