ci: pin the declared dependency floors exactly in the floors job

[lesnik512]

The floors job now installs each declared floor exactly, so a floor with no wheel fails the resolve instead of passing on a higher version.

The gap

--only-binary PyYAML doesn't make a floor with no wheel fail. It makes that version ineligible, and --resolution lowest-direct then climbs to the lowest version that has a wheel, then passes. This is how PyYAML>=6 resolved 6.0.1 and 6.0.2 on 3.12 and 3.13 while the job stayed green (#135). uv has no option that turns the climb into an error. Its maintainers treat skipping versions without wheels as intended (astral-sh/uv#16186).

The fix

scripts/floor_constraints.py reads [project.dependencies] and every [project.optional-dependencies] extra from pyproject.toml and prints one exact pin per declared floor. A lower bound of >=, ~= or == becomes ==, and each marker is kept verbatim so uv evaluates it for the interpreter being tested:

PyYAML==6; python_version < "3.12"
PyYAML==6.0.1; python_version == "3.12"
PyYAML==6.0.2; python_version == "3.13"
PyYAML==6.0.3; python_version >= "3.14"

The floors job passes this file as --constraints together with the existing --only-binary PyYAML. A dependency with no lower bound fails the generator, because an exact pin can't be derived for it.

This follows what larger projects do: sunpy and jwst (minimum_dependencies), napari (tox-min-req), scikit-image and scikit-learn all generate exact minimum pins rather than relying on a lowest resolution. None of those I checked also enforces wheel-only.

packaging and tomli join the dev group because the tests import them. tomli rather than tomllib keeps the script importable on 3.10 without a version-gated import. The core package stays zero-dependency.

Verified locally

I ran the CI steps with Linux wheels (--python-platform x86_64-manylinux_2_28):

case	result
3.10 to 3.14 as declared	installs 6.0, 6.0, 6.0.1, 6.0.2, 6.0.3: each declared floor exactly
3.12 with the floor planted back to PyYAML>=6	No solution found ... pyyaml==6.0 has no usable wheels, exit 1

just lint-ci passes, just test-ci passes 1482 tests at 100% coverage, and the generator's tests pass on 3.10.

Known limits

• Overlapping markers. Two declarations of one dependency are merged only when their marker text is identical; the higher floor wins. Different markers that overlap for one interpreter, such as a core a>=1 and an extra's a>=2; python_version >= "3.12", would give two contradictory pins. Nothing in this repo does that.
• uv sync repos. uv sync takes no constraints flag. Repos that install with uv sync --no-install-project would have to move to uv pip install, or set [tool.uv] constraint-dependencies, before they can use this.