Add Python 3.10 support

[aleehedl]

Currently, Python 3.10 is not supported. Is it on the roadmap?

For what I know, contracts library may be an issue because it hasn't been updated in years and now some warnings turn into errors.

[paulboocock]

Yep, it's definitely on the roadmap. I hope we will find time to look at this during Q4 2021.

We're open to PRs if you (or anyone reading this) would like to contribute to this tracker.