add typing._Final

[tungol]

This is the subset of typing module internal base class inheritance that's stable over all supported versions of python.

This probably isn't worth doing either. But like the maximalist version of this MR, I wanted to work out what it would look like, and once I did that I feel like it makes sense to push it up to generate a record of that and let other people see it too.

I know I've pushed up a bunch of MRs lately which are really scraping the bottom of the barrel, metaphorically speaking, in the name of accurate inheritance. So if it's not too weird, I wanted to say that I hope that it's not too much of a pain for the maintainers here for me to do that. I appreciate the work that you all do and I don't want to be making it harder for you.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pydantic (https://github.com/pydantic/pydantic)
+ pydantic/v1/typing.py:41: error: Unused "type: ignore" comment  [unused-ignore]
- pydantic/_internal/_typing_extra.py:410: error: Module has no attribute "_Final"; maybe "Final"?  [attr-defined]