Structure project according to Python conventions

[maxbechtold]

This includes

• [x] ~package~ module structure
• [x] separate source and test directories

and maybe also

• [x] separate resource and script directories
• [x] revised code structure enforced by linting rules

[maxbechtold]

This offers a lot of good hints: https://docs.python-guide.org/writing/structure/

[maxbechtold]

Resolved