uv (pip replacement) support

[mikepink]

uv is a drop-in replacement for pip which runs very quickly. We've replaced pip with uv locally and have noticed a significant speed up in our Python build time in our development and CI environments. By adopting support for uv, we anticipate our Heroku builds would complete ~5 minutes more quickly.

Because uv is a drop-in replacement, we found adoption to be very straightforward in our case. pip install -r requirements.txt becomes uv pip install -r requirements.txt. The promise of significantly improved performance with a very simple migration path was enough to convince us to give it a try internally.

Happy to attempt a PR if that would be welcome/well received.