Add options to tkinter widgets (#4363)

In tkinter, `widget['foo'] = bar` and `widget.config(foo=bar)` do the same thing, but they will now type-check differently: the `widget['foo'] = bar` syntax allows 'foo' to be any string (e.g. a variable, not necessarily a Literal) and bar to be any object, while `widget.config(foo=bar)` checks the existence of the option and the type of bar. Similarly, cget takes a Literal argument but __getitem__ takes a string. 

Testing script can still be found at c42a72c53e

requirements-tests-py3.txt

@@ -5,4 +5,4 @@ flake8==3.8.3
 flake8-bugbear==20.1.4
 flake8-pyi==20.5.0
 isort[pyproject]==5.1.1
-pytype>=2020.07.24
+pytype>=2020.07.30