Test st.from_type() on builtin generics from Python 3.9

[Zac-HD]

Python 3.9 is coming out soon! Happily, we're already running it in our CI - so anything that worked before still works on the new Python. There are however also new features, like PEP 585 (type annotations using builtins as generics).

As Guido highlights, we should also check that everything works with from __future__ import annotations - that's not new in 3.9 but will become the default behaviour in 3.10!

I don't think we actually need any new logic to handle this, but we should really add some py39-only tests to make sure it keeps working 😁

[Zac-HD]

WIP at master...Zac-HD:pep585-annotations and it turns out that we do need some new logic after all. Ah well.