TYP: issues with type-checking against numpy 2.1

[neutrinoceros]

Type-checking is now failing against NumPy 2.1 (example logs)

We're seeing two errors

cmasher/utils.py: note: In function "_get_cmap_lightness_rank":
cmasher/utils.py:167: error: Unsupported left operand type for - (Never)  [operator]
cmasher/utils.py: note: At top level:
cmasher/utils.py:1554: error: Unused "type: ignore" comment  [unused-ignore]
Found 2 errors in 1 file (checked 63 source files)

The first one I think is a bug in NumPy, which I'll report upstream if it hasn't been done already.
The second one (Unused "type: ignore" comment) would be easy to solve (remove the unused comment), but unfortunately the comment is still needed on Python 3.9, at it isn't supported by NumPy 2.1. Fixing this part will require either dropping Python 3.9 (which is maybe a bit radical, unless we see it as a tradeoff for adding 3.13 (which I intend to do very soon)), or temporarily disabling this warning in mypy's configuration (which sounds much more reasonable).

[neutrinoceros]

After further testing, the first error, while unclear, appear to be due to a conflict between mypy's configuration (python_version = '3.9') and running against NumPy 2.1
Since both problem are solved by dropping Python 3.9, I'll just go with it.