ci: Add Python 3.10 to testing

[matthewfeickert]

This PR adds CPython 3.10 to the testing matrix used in CI and adds CPython 3.10 support to the PyPI metadata. It additionally sets the oldest CPython version tested on macos-latest to be Python 3.7 given that the upcoming change in macos-latest to a macOS-11 image will drop support for all EOL Pythons, which is Python 3.6 and older (c.f. https://github.com/actions/virtual-environments/issues/4060).

That's the total effect of this PR. Internally in the GHA CI workflow it does two other things:

• It uses the build matrix include option over the exclude option to simplify the "ends" of the Python versions tested on macos-latest. By removing macos-latest from the os list and then using include one only needs to specify macos-latest and the python versions only two numbers need to be manually maintained in the CI: the oldest Python to be tested on macOS and the newest.
• The Python version numbers are quoted as without quotes YAML will treat 3.10 as a float and interpret it as 3.1 which is certainly not desired. :)

[matthewfeickert]

Hi @jiffyclub :wave:. This PR is ready for review. Let me know if you have any questions. :+1:

[jiffyclub]

Thank you!