feat/uv support

[blast-hardcheese]

Why

Resolve #244

What changed

• Adding python3-uv backend

Test plan

CI, manual testing

[ryantm]

Yay uv. We'll only use it for hatchling.build for now, so this seems safe and looks good to me!

[blast-hardcheese]

I relaxed the requirement for hatchling.build as uv apparently only uses it as the default, there are CLI flags to override it. I altered it to "so long as Poetry is not configured", since we only support the three currently.

[AYA-123]

When installing python packages, will the default method of installation be via uv?

[AYA-123]

And using https://github.com/astral-sh/ruff for linting and code formatting (by the creators of uv), would also yield similarly fast and beneficial results.

[blast-hardcheese]

We're gradually rolling uv out to new Repls, starting with the agent. If you'd like to use it today, the easiest way is in a new repl:

1. Make sure you have a python nixmodule installed (Either fork the Python template, run python at the shell and type Y to add the module, or use the Configuration pane to select a version of Python to use).
2. upm add -l python3-uv add flask will create a new uv project, including making .pythonlibs a full virtualenv, and install flask. Subsequent operations inside Replit will correctly detect uv and use it.
3. Occasionally, after rebuilding your Repl environment (for instance when changing nix channels), you'll notice that uv will delete and recreate your .pythonlibs directory.
   This is because the nix store path for the python executable may have been removed in the upgrade.

You can run replit-fsck after an upgrade to check whether any links or shebangs need to be patched, which should maintain your .pythonlibs even through an upgrade. This'll eventually be part of the full flow before we take uv GA.

Thanks for the interest!