This pull request relaxes PEP 586 so that type checkers are now expected
to maintain backwards compatibility on a best-effort basis, rather than
a mandatory basis.
It also rearranges the order in which information is presented: the
section now opens with and focuses on an example of what a too-disruptive
inference strategy would look like.
This commit adds a section discussion how Literal types interact with
Final from PEP 591, if both PEPs are accepted. In short, it states
that type checkers are expected to understand that assignment statements
of the form `var: Final = value` are a way of declaring `var` to
effectively be of type `Literal[value]` in certain cases.
Guido van Rossum
Michael Lee
Vincent Poulailleau
Serhiy Storchaka