Fabric upgrade needed

[vossisboss]

Per Ben Dickinson from Torchbox in our internal project logs:

"We should upgrade to modern version of fabric (or even better, switch to invoke, because we don't need SSH). This might also be a good opportunity to drop the Makefile and move everything into the fabfile.py so it's a bit less fragmented.

This should also unblock us using Python>=3.9, with which our current fabric3==1.14.post1 version isn't compatible."

We'd appreciate any proposals or ideas that backend-oriented folks have for improving our tooling.