CI: Add python 3.10.0-rc2 to GitHub Actions

[FollowTheProcess]

How would this feature be useful?

With the release of python 3.10 around the corner, it's probably a good idea to run the release candidate against nox in CI to uncover any weirdness.

Describe the solution you'd like

Add 3.10.0-rc2 to matrix.python_versions in ci.yml

At the moment, conda does not support 3.10 so this would just be for standard python for now. You can see this running (and passing) in my fork here: https:/FollowTheProcess/nox/runs/3559269771?check_suite_focus=true

Describe alternatives you've considered

I guess wait until 3.10 is released officially but seems like a no brainer to get ahead of the game!