Add mypy typings to project compatible with python 3.10 + run unit tests in CI

[jacobdr]

This PR adds mypy type checking to all of our internal code -- whereas #8 added typing for our external code

I used pretty modern type syntax, therefore this PR now requires project developers (and our release machinery) to use Python 3.10 specifically (though really we could support 3.10+, I wanted to KISS for now)

[samwillis]

Happy to be tied to 3.10 for now and add these typings. All looks good, once the conflicts are fixed will merge.

[samwillis]

We should update 'DEVELOPMENT.md' to note the minimum python version.