Migrate venv management and requirements to uv

[nils-werner]

This MR migrates the manual management of requirements using a requirements.txt file to uv.

This completely gets rid of manual venv management, so instead of

python -m venv .env
source .env/activate
pip install -r requirements.txt
python manage.py

You can simply let uv do all of that and run

uv run manage.py

And allows for very convenient dependency management, e.g. to later bump a dependency, all you have to do is

uv lock --upgrade-package <pkg>

or upgrade all of them

uv lock --upgrade

and many more quality of life improvements.

Heroku supports uv out of the box and even mentions it in a recent blogpost.