New release #799

I'm hoping to cut a new release soon. Some details:

• I'd like to wait for backport ParamSpecArgs/Kwargs #798 and the typing-extensions equivalent of bpo-43766: Implement PEP 647 (User-Defined Type Guards) in typing.py cpython#25282 to make it in, so that we'll have parity with Python 3.10.0.
• I'll release both typing and typing-extensions under the same version because that seems to be tradition. We haven't had a lot of changes to typing recently (it's only relevant to people stuck on 2.7 or 3.4), but we should at least get Update setup.py #784 released.
• There is no documentation in the repo on the versioning schema, but I think the release should be called 3.10.0 to imply that it contains all the typing features that will be in Python 3.10.0. Later, if we do a release that fixes a bug but doesn't add new features, we'll call it 3.10.0.1, but if we do a release that adds support for another new type system feature, we'll call it 3.11.0 (or whatever it ends up being). @gvanrossum is this right? I'll also add a note to the README explaining the versioning schema.