GitHub Actions: Add Python 3.10 to the testing

[cclauss]

https://pythoninsider.blogspot.com/2021/10/python-3100-is-available.html

"3.10" must be quoted in yaml... https://dev.to/hugovk/the-python-3-1-problem-85g

[cjolowicz]

Brilliant, thanks for doing this! We'd still need changes in the generated project, in the GA workflow and noxfile.py, under {{ cookiecutter.project_name }}.

[cjolowicz]

Can you rebase? That should fix the test suite.

[cclauss]

Rebased.

[cjolowicz]

Thank you!