TYP: resolve mypy ingores in core/indexing.py
xref https://github.com/pandas-dev/pandas/issues/37715

Notes: `axis` has to be passed as an argument to `_tupleize_axis_indexer` in order to assert that it is not `None`. Since this kind of decouples it from the `_LocationIndexer` class, I made it a standalone utility function.
Thanks @iasoon 