Fix two static type errors reported by mypy 2.0.0

[seisman]

The new mypy v2.0.0 reports two new errors:

mypy pygmt
pygmt/clib/session.py:113: error: Need type annotation for "GMT_CONSTANTS" (hint: "GMT_CONSTANTS: dict[<type>, <type>] = ...")  [var-annotated]
pygmt/figure.py:96: error: Need type annotation for "SHOWED_FIGURES" (hint: "SHOWED_FIGURES: list[<type>] = ...")  [var-annotated]
Found 2 errors in 2 files (checked 128 source files)
make: *** [Makefile:74: typecheck] Error 1

xref: https://github.com/GenericMappingTools/pygmt/actions/runs/25474333778/job/74744492600

[weiji14]

Was gonna recommend using parallel type checking, but then remembered this runs on ubuntu-slim with just 1 CPU 😆