Return minorticks as array, not as list.

[anntzer]

The return type changed accidentally when remove_overlapping_locs was
introduced (except for NullLocator, which always returned a plain list).
Make the API consistent with majorticks, which are (mostly) always
returned as an array.

Modifying the test in test_colorbar is so-so, but that's the only place,
AFAICT, that currently tests anything with get_ticklocs(minor=True),
so...

Closes #19559.

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).

[timhoffm]

Should this be 3.5?

[anntzer]

Technically that's a regression in 3.1, but again either way is fine for me.

[timhoffm]

This can either be considered a bug fix or an API change. I've decided not to push this into 3.5 on the last minute. This has been around for some time. One release more does not hurt a lot.