Modern python structure

[LecrisUT]

• [x] PEP621
• [x] src-layout
• [x] move the executable to script installation
• [x] expose fypp to be run as python -m fypp
• [ ] Modern pytest structure (Postponed for later)
• [ ] Better Sphinx support (Postponed for later)
• [x] Github action CI

[LecrisUT]

@aradi Could you enable the actions for this PR?