[Typing-sig] PEP 591 discussion: final

I've submitted PEP 591 to the PEPs repository. This PEP proposes a "final" qualifier to be added to the ``typing`` module---in the form of a ``final`` decorator and a ``Final`` type annotation---to serve three related purposes: * Declaring that a method should not be overridden * Declaring that a class should not be subclassed * Declaring that a variable or attribute should not be reassigned Discussion on the PEP happens here in typing-sig@. Minor things can be fixed directly as PRs. Here is the full PEP draft: https://www.python.org/dev/peps/pep-0591/ A reference implementation exists in mypy, whose documentation is here: https://mypy.readthedocs.io/en/stable/final_attrs.html (though the PEP takes precedence) -sully