Properly set up Python sub-projects in the repo

[DMozhevitin]

Clarification and motivation

We have several Python sub-projects in the repository, namely docker/build/package.py and baking/src/tezos_baking.

Over time their size has increased, and it would be nice to package them in the idiomatic way (e.g. using Poetry), create a nix shell with necessary environment for Python development, and use other best practices.

Acceptance criteria

• Python sub-projects in the repo are structured using the idiomatic practices and suitable tools