Phalanx development now uses uv

Hello everyone,

Following very successful experiences with our other services, we are switching to uv to manage the Phalanx Python code. If you are doing Phalanx development, either as an environment administrator or an application developer, you will need to have uv installed.

Nothing else should (hopefully) change in the development workflow apart from that requirement.

There are lots of ways to install uv, but the easiest way for most people will probably be either pip install uv or (for macOS users already using Homebrew) brew install uv.