ENH: Vendor typing_extensions
I’ve discussed this with @simonjayhawkins here and there but never had a formal issue opened, hence this :-)

I think we need to vendor typing_extensions to realistically be able to unpin mypy from where it is now.  The main thing I think we need is to unlock Protocols; these aren’t added until Python 3.8 but as of mypy 0.75 seem to be the documented solution to a lot of the Mixin problems that we face:

https://mypy-lang.blogspot.com/2019/11/mypy-0.html

There are some other useful things like Literal, Final and TypedDict that this would unlock as well, to at least hold us over until 3.8 is the minimum supported version