gh-91243: Add typing.Required and NotRequired (PEP 655) (GH-32419)

I talked to @davidfstr and I offered to implement the runtime part of PEP 655
to make sure we can get it in before the feature freeze. We're going to defer
the documentation to a separate PR, because it can wait until after the feature
freeze.

The runtime implementation conveniently already exists in typing-extensions,
so I largely copied that.

Co-authored-by: David Foster <david@dafoster.net>