Add uv export --format requirements.txt

[charliermarsh]

Summary

The interface here is intentionally a bit more limited than uv pip compile, because we don't want requirements.txt to be a system of record -- it's just an export format. So, we don't write annotation comments (i.e., which dependency is requested from which), we don't allow writing extras, etc. It's just a flat list of requirements, with their markers and hashes.

Closes #6007.

Closes #6668.

Closes #6670.

[charliermarsh]

Wait... I thought this was a cool feature, but I don't think there's any way to "activate" these from uv pip install or pip install...? So we might need the export API to accept extras, to tell us which to include.

[konstin]

Looks good! Only some nitpicks

[konstin]

Setuptools already requires wheel nowadays (https://setuptools.pypa.io/en/latest/build_meta.html#how-to-use-it)

Suggested change

	requires = ["setuptools>=42", "wheel"]
	requires = ["setuptools>=42"]

[charliermarsh]

Oh nice. I will change this separately since it's in all tests.

[konstin]

Could you add a comment on why we expect the markers to disappear? The combination of python_version > '3.11' and requires_python = ">=3.12" is hard to spot.

[zanieb]

Can we write a bit more here? What are the implications for a user running an export here? e.g.

Do not update the uv.lock before exporting.

If a uv.lock does not exist, uv will exit with an error.

Did you copy these from somewhere? I thought I touched most of these up to say stuff like that.

[charliermarsh]

The LockArgs one.

[zanieb]

Ah, that one is the simplest because it's for uv lock. I'd look at the styling in uv sync

      --locked
          Assert that the `uv.lock` will remain unchanged.
          
          Requires that the lockfile is up-to-date. If the lockfile is missing or needs to be updated, uv will exit with an error.

      --frozen
          Sync without updating the `uv.lock` file.
          
          Instead of checking if the lockfile is up-to-date, uses the versions in the lockfile as the source of truth. If the lockfile
          is missing, uv will exit with an error. If the `pyproject.toml` includes changes to dependencies that have not been included
          in the lockfile yet, they will not be present in the environment.

[helderco]

Pretty cool, this will help migrate to uv.lock because users will have a way to revert easily if needed. 🎉