Fix type annotations in pandas.core.indexing
Part of #25882 

Current errors are:

    pandas\core\indexing.py:1487: error: Incompatible types in assignment (expression has type "Type[Exception]", base class "_NDFrameIndexer" defined the type as "Type[KeyError]")
    pandas\core\indexing.py:1783: error: Incompatible types in assignment (expression has type "str", base class "_NDFrameIndexer" defined the type as "None")
    pandas\core\indexing.py:2056: error: Incompatible types in assignment (expression has type "str", base class "_NDFrameIndexer" defined the type as "None")
    pandas\core\indexing.py:2058: error: Incompatible types in assignment (expression has type "Type[IndexError]", base class "_NDFrameIndexer" defined the type as "Type[KeyError]")