CLN: Clean-up numeric index dtype validation
The use of a dict with `_typ` attributes as keys in `NumericIndex._validate_dtype` makes it complicated to subclass numeric indexes and change the subclass' `_typ` attribute.

This PR makes the dtype validation in `NumericIndex._validate_dtype` not dependent on the value of the `_typ` attribute, making subclassing more robust.
thanks @topper-123 