Handle Index without arguments, subclassing DataFrame and register accessor arguments

[Dr-Irv]

• Closes register_dataframe_accessor causes "Untyped class decorator obscures type of class; ignoring decorator" #906
• Closes var-annotated error for Series and Index after upgrading to 2.0.3+ #826 (previously closed but added test to detect this)
• Closes Subclassing pandas DataFrame results in mypy misinterpreting indexers return #855 (return Self for DataFrame methods - has lots of changes)
• Tests added:
  • test_frame.py:test_frame_subclass()
  • test_frame.py:@pd.api.extensions.register_dataframe_accessor("geo")
  • test_indexes.py:test_frame_subclass()

[twoertwein]

Thank you @Dr-Irv !