Error out from workflow on missing interpreter

[mayeut]

The lint and docs workflow will fail silently if there's a
mismatch between the python version setup in the workflow and
the one required in noxfile.py.

Run nox with --error-on-missing-interpreters to error out rather
than silently skipping the session.

[mayeut]

Here are examples with current main branch with an interpreter mismatch:

• https://github.com/mayeut/packaging/runs/5610610984?check_suite_focus=true#step:5:8
• https://github.com/mayeut/packaging/runs/5610611004?check_suite_focus=true#step:5:8

Here are examples with this PR with an interpreter mismatch:

• https://github.com/mayeut/packaging/runs/5610637351?check_suite_focus=true#step:5:9
• https://github.com/mayeut/packaging/runs/5610637366?check_suite_focus=true#step:5:9

[mayeut]

The required "Ubuntu / 3.9" won't run as I only modified the docs & lint workflows.

[pradyunsg]

I've removed the Ubuntu / 3.9 required check.