[python] Use _index_factory in AxisIndexer class

[jp-dark]

Replace pd.Index with the user provided _index_factory when construction the cached var/obs indices.

[thetorpedodog]

A couple minor things that I think are technically only relevant to static analysis. It is probably worth running a tiledbsoma pipeline with the somacore dep pointed at this change to make sure it works end-to-end.

[jp-dark]

It is probably worth running a tiledbsoma pipeline with the somacore dep pointed at this change to make sure it works end-to-end.

Tested this locally and all tests passed.