Fix mask lookup in fill_between for NumPy 1.24+

[QuLogic]

PR Summary

I did not bother adding any new tests, as the existing tests will already break with the latest nightly NumPy wheels.

Fixes #24106

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

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

[oscargus]

Semi-related: there are still uses of getmask in other parts of the code. Would it be worthwhile revisiting those and replace with getmaskarray? I assume that most of the time we do want an array and that many of those may break with the "wrong" input data?

[QuLogic]

Possibly, but none of them triggered a test failure with latest NumPy wheels, other than this one.