1. Proxy inside the company network that intercepts every request to pypi? Doesn't work well due to https, I guess.
2. Replacing pypi in your project description with your own mirror? Might work, but at least slows down every build outside of your network. Also needs to be replicated for npm, cargo, maven, docker, ...
3. Start a userspace wrapper before starting the build that transparently redirects every request. That would be the best solution, IMO. But how do I intercept https requests of child processes? Technically, it must be possible, but is there such a tool?