Fix type annotations for pandas.core.indexes.datetimes
Part of #25882

Current errors are:

    pandas\core\indexes\datetimes.py:7: error: Module 'pandas._libs' has no attribute 'join'
    pandas\core\indexes\datetimes.py:89: error: Definition of "_data" in base class "DatetimeIndexOpsMixin" is incompatible with definition in base class "Index"
    pandas\core\indexes\datetimes.py:89: error: Definition of "_data" in base class "DatetimeIndexOpsMixin" is incompatible with definition in base class "DatetimelikeDelegateMixin"
    pandas\core\indexes\datetimes.py:89: error: Definition of "map" in base class "DatetimeIndexOpsMixin" is incompatible with definition in base class "Index"
    pandas\core\indexes\datetimes.py:89: error: Definition of "_format_with_header" in base class "DatetimeIndexOpsMixin" is incompatible with definition in base class "Index"
    pandas\core\indexes\datetimes.py:89: error: Definition of "isin" in base class "DatetimeIndexOpsMixin" is incompatible with definition in base class "Index"
    pandas\core\indexes\datetimes.py:1115: error: "Callable[[Any], Any]" has no attribute "fget"
    pandas\core\indexes\datetimes.py:1116: error: "Callable[[Any], Any]" has no attribute "fget"
    pandas\core\indexes\datetimes.py:1117: error: "Callable[[Any], Any]" has no attribute "fget"
