TYP: mypy [attr-defined] error in `pandas/core/indexes/numeric.py`
This is one of the errors after unpin mypy. 

```
pandas/core/indexes/numeric.py:68: error: "Type[NumericIndex]" has no attribute "_default_dtype"  [attr-defined]
pandas/core/indexes/numeric.py:69: error: "Type[NumericIndex]" has no attribute "_default_dtype"  [attr-defined]

```