fix(datasource/python): mark skeleton coordinate space as valid
fix(datasource/python): mark skeleton coordinate space as valid

Previously, the coordinate space from a Python skeleton datasource was incorrectly marked "invalid",
which kept Neuroglancer from rendering skeletons if no other layers were present.

google/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer