Clean-up

[oktaal]

convert, Tree and InjectableFile seem to be somewhat useless at the moment now. They might be removed.

[oktaal]

(Ok InjectableFile might become useful when #8 is implemented)

[oktaal]

Use pip-compile from pip-tools to clean up the requirements. Maybe look into how the dev specific requirements (linter) can be retained as well.

[jgonggrijp]

You can use two requirements.in files, one for strict dependencies and one for dev-only dependencies. Or you can just throw all dependencies into a single requirements.in and accept a little bit of overhead in production.