Run CI on 3.9.5

[tomchristie]

Asyncio on Python 3.9.7 has some grubby bug that's causing our CI to hang.

I don't really understand the interaction there, but right now I don't really care either, I'd just like to see the tests running properly. I'm assuming that a `"3.9.5" specifier here will do what we'd expect, but let's find out, eh?

Some things that'd be useful...

• Doc'ing up exactly what the 3.9 CPython asyncio bug is and why it's causing our tests to hang.
• Where does GitHub document which versions are available on it's CI platform?

Anyone's want to chime in on these in this thread?

[tomchristie]

I think the valid Python versions are listed here... https://github.com/actions/python-versions

[sondrelg]

Yep specifically here https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

[tomchristie]

cough 3.9.5

[tomchristie]

Right, once 3.9.8 is available, let's give that a try and switch back to a plain "3.9" if it's working okay again then.
None of that jankiness is anything to do with httpx, but it would still be useful to comprehensively understand why the behaviour was hanging.