Convert from flit to poetry

The purpose of this MR is to convert from the Flit build system to the Poetry build system.

Flit is a really nice and simple Python build system, up-to-date with the most recent Python build PEPs. Poetry is the "batteries included" build system, and it is the only modern Python build system that supports lockfile generation. It's this lockfile system which makes Poetry builds reproducible, and this is why Nix relies on Poetry.

Whether or not we proceed to use Nix, we should consider adopting Poetry, because it will make it much easier to pin and unpin our dependencies and ensure we are all using the same versions of our dependencies.