Use src/ folder for package

[sepro]

Someone recently pointed out in one of my packages that currently it is considered best practice to have a src/ folder and include the module's code there. Which is correct and some details can be found here. Two users also indicated issues when installing packages using alternative tools (e.g. uv, ... though I haven't been able to replicate that, take that with a healthy dose of skepticism).

This would require moving the statannotations folder to src/statannotations and making the corresponding changes to setup.py and config for unit-tests and github actions.

Happy to provide a PR, if there is interest and it wouldn't cause issues I haven't considered.