Silence a mypy warning with mypy v1.12.0

[seisman]

The "Static Type Checks" workflow started to fail due to the newly released mypy v1.12.0 (https://github.com/GenericMappingTools/pygmt/actions/runs/11330529462).

I don't know if it's an indented change or a newly introduced bug, and I don't have a solution for it, so just ignore the mypy error.

mypy pygmt
pygmt/datasets/load_remote_dataset.py:444: error: Value of type "Literal['gridline', 'pixel'] | None" is not indexable  [index]
Found 1 error in 1 file (checked 106 source files)
make: *** [Makefile:74: typecheck] Error 1

[weiji14]

Yeah, None can't be indexed, and we do need to keep the None option for automatic determination of gridline/pixel registration (handled at L427-L429). Not sure how to workaround the typechecker here too, so ok to ignore for now.