Switch to the package manager uv

[schneems]

We should update the app to use UV to match buildpack best practice recommendations

remote: ! Note: We recently added support for the package manager uv: 
remote: ! https://devcenter.heroku.com/changelog-items/3238 
remote: ! 
remote: ! It's now our recommended Python package manager, since it 
remote: ! supports lockfiles, is faster, gives more helpful error 
remote: ! messages, and is actively maintained by a full-time team. 
remote: ! 
remote: ! If you haven't tried it yet, we suggest you take a look! 

[edmorley]

@schneems Yeah this is already tracked in this GUS epic.