Add support for PEP 695

[charliermarsh]

Summary

PEP 695 adds new syntax for type parameters and generics. A few examples:

class ClassA[T: str]:
    def method1(self) -> T:
        ...

type ListOrSet[T] = list[T] | set[T]

There are a few pieces of work involved in this:

• Adding these elements to the AST.
• Adding these elements to the parser (especially the type soft keyword).
• Adding these elements to the visitor, and to any rules that do custom visiting.
• Adding these elements to our semantic model (need to look into how scoping and resolution works).

This will be Ruff's first time undergoing a significant grammar and syntax change (apart from adding structural pattern matching), so it's a great opportunity to set the project's standards for keeping up with Python's evolution.

Specifically, the final Python 3.12 release is expected on 2023-10-02. We should aim to support PEP 695 by that date at the latest.

[charliermarsh]

I will take ownership of this work.

[charliermarsh]

\cc @zanieb who's working on this :)

[zanieb]

Progress on the Rust Python parser can be tracked at RustPython/Parser#82