fix(deps): bump upet lower bound to 0.3.0 for the metatomic extra

[lil-lon]

Summary

Fixes the failing metatomic test jobs in CI.

• lowest-direct: upet 0.2.0 fails to import with recent warp-lang (Pin warp-lang version to pyproject.toml lab-cosmo/upet#147).
• highest: uv falls back to upet 0.2.6, whose metatrain cannot read the latest pet-mad checkpoint.
• upet 0.3.0 fixes both.

Checklist

Before a pull request can be merged, the following items must be checked:

• Doc strings have been added in the Google docstring format.
• Run ruff on your code.
• Tests have been added for any new functionality or bug fixes.

[CompRhys]

thanks!