Support pathlib Paths in Loadable

[friggog]

#2099

[mikedh]

Thanks for the fix! Just to double check it shouldn't have raised a runtime TypeError in this case, it's just that pylance/etc gets mad at it?

# pre this fix
In [4]: import trimesh

In [5]: from pathlib import Path

In [6]: trimesh.load(Path('models/featuretype.STL'))
Out[6]: <trimesh.Trimesh(vertices.shape=(1722, 3), faces.shape=(3476, 3), name=`featuretype.STL`)>

[friggog]

Yeah exactly, not runtime error, but pylance and linters like Mypy would complain