Fix type annotations in pandas.core.indexing

[vaibhavhrt]

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]")

[vaibhavhrt]

PR coming soon