Update trove classifiers and build matrix

[tonybaloney]

Fixes #minor

This PR updates the trove classifiers for each of the published packages. They all specify on 3.7, but the build matrix passes on 3.8.

The current stable version of Python is 3.10 so this PR adds those versions to the build matrix and adds the classifiers.

Note: It might be also worth setting python_requires=">=3.7" in each. This would mean that users with 3.6 will get the older version