`numba` not declared as an optional dependency - not tested against

[jGaboardi]

numba is an optional dependency, but it is never actually stipulated as such in pyproject.toml or anywhere else I can find. Moreover, it is not present in any testing environment. This may be a moot point if implementing the shapely version of minimum_bounding_circle(), etc. negates the need for even using numba.

cc @ljwolf @martinfleis

xref:

• skyum raises NameError when numba not installed #81
• Move to shapely 2 for minimum bounding rectangle (rotated) and minimum bounding circle #75
• HAS_NUMBA left undefined #70
• add numba-fied version of skyum code #56