Ensure TwoSlopeNorm always has two slopes

[dstansby]

PR Summary

Fixes #24283.

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst

[jklymak]

Wouldn't it be better to just error out here?

[dstansby]

Given this is in the autoscaling part (the user hasn't explicitly requested a vmin/vmax), I'd say it's better to choose some sensible vmin/vmax and give the user some sort of plot - if they then want to change the vmin/vmax that's at least a starting point to go from.

[tacaswell]

This needs atleast a whats new and or maybe a behavior change note.

[jklymak]

This seems good to me after a bit of documentation. Ping for a review if it gets buried

[dstansby]

Added a behaviour change note as I didn't think this warranted a what's new - happy to add or move to what's new though if anyone disagrees.

[jklymak]

This looks good to me. Thanks, and sorry for misunderstanding the intention.

[tacaswell]

Restarted the failed job, looked unrelated (a subprocess call for blitting with tk).

[tacaswell]

Anyone can merge on green.