Use Nix to test against different python versions?

[teodorlu]

Right now, I don't have good feedback on whether I break any of the supported Python versions. Nix could help solve that.

@jlesquembre was kind enough to provide nix declarations for running Hotload on different python versions: https://github.com/jlesquembre/hotload/blob/master/flake.nix

@tingstad - perhaps we can run CI tests against different Python versions?