TYP: Distinguish use of numpy.dtype and ExtensionDtype

[twoertwein]

Index inherits from IndexOpsMixin which defines self.dtype as a property of Union[np.dtype, "ExtensionDtype"]. There are a few places where either numpy.dtype or ExtensionDtype are expected, but not both:

pandas/core/indexes/base.py:6857: error: Item "dtype[Any]" of "Union[dtype[Any], ExtensionDtype]" has no attribute "categories" [union-attr]
pandas/core/indexes/base.py:6857: error: Item "ExtensionDtype" of "Union[dtype[Any], ExtensionDtype]" has no attribute "categories" [union-attr]
pandas/core/arrays/categorical.py:2649: error: Item "dtype[Any]" of "Union[dtype[Any], ExtensionDtype]" has no attribute "construct_array_type" [union-attr]
pandas/core/indexes/period.py:429: error: Item "dtype[Any]" of "Union[dtype[Any], ExtensionDtype]" has no attribute "resolution" [union-attr]
pandas/core/indexes/period.py:429: error: Item "ExtensionDtype" of "Union[dtype[Any], ExtensionDtype]" has no attribute "resolution" [union-attr]
pandas/core/indexes/period.py:482: error: Item "dtype[Any]" of "Union[dtype[Any], ExtensionDtype]" has no attribute "resolution" [union-attr]
pandas/core/indexes/period.py:482: error: Item "ExtensionDtype" of "Union[dtype[Any], ExtensionDtype]" has no attribute "resolution" [union-attr]
pandas/core/indexes/interval.py:531: error: Item "dtype[Any]" of "Union[dtype[Any], ExtensionDtype]" has no attribute "subtype" [union-attr]
pandas/core/indexes/interval.py:531: error: Item "ExtensionDtype" of "Union[dtype[Any], ExtensionDtype]" has no attribute "subtype" [union-attr]
pandas/core/indexes/interval.py:747: error: Item "dtype[Any]" of "Union[dtype[Any], ExtensionDtype]" has no attribute "subtype" [union-attr]
pandas/core/indexes/interval.py:747: error: Item "ExtensionDtype" of "Union[dtype[Any], ExtensionDtype]" has no attribute "subtype" [union-attr]
pandas/core/internals/blocks.py:1613: error: Item "dtype[Any]" of "Union[dtype[Any], ExtensionDtype]" has no attribute "na_value" [union-attr]

[twoertwein]

All of the above cases will need ignore statements. In python 3.10, a few ignore statements can be avoided by using typing.TypeGuard.

I'm closing this issue as ignore statements can only be added once the "CI mypy" starts complaining, which will happen when cache_readonly is typed.