MyPy 1.9 has dropped support for Python 3.7.

n.b. Cancelled all but the fast tests, I think that's enough for this.

Checklist

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• This is a dependency change, no changes to documentation or tests required.