Support Python 3.10

[fabiencelier]

Python 3.10 has been released

The wheel is cross Python versions so it only need to be tested in 3.10.
The conda package is still one per Python version so a package for 3.10 should be released

[fabiencelier]

Depends on actions/setup-python#249

[hugovk]

Python 3.10.0 is now available: actions/setup-python#249 (comment)

Edit: but Conda isn't ready yet: conda/conda#10969 / ContinuumIO/anaconda-issues#12669