CI: Clean up nightly build a bit
CI: Clean up nightly build a bit

No need for a matrix strategy for a single element, so remove. We're
only running single jobs on these, so remove all the places where we're
trying to use `matrix.python-version` to make unique.

Also bump the nightly tests environment to Python 3.10 while we're here.

Unidata/MetPy,Unidata/MetPy