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