Update testing and CI: tox-uv, lowest-resolution oldestdeps, role-based job names

[tepickering]

This updates the testing infrastructure to match astropy/astroplan#641, adapted to specreduce.

Testing infrastructure

• tox-uv. tox now uses tox-uv, which tox installs automatically through requires. pip-specific settings are replaced with their uv equivalents: {list_dependencies_command} instead of pip freeze, and UV_EXTRA_INDEX_URL with unsafe-best-match so devdeps can mix nightly and PyPI packages. The git-installed devdeps (specutils, synphot) use pkg @ git+... syntax.
• oldestdeps uses uv's lowest resolution. Every package is installed at the lowest version allowed, instead of relying on hand-written pins.
  • Minimums for direct dependencies come from pyproject.toml.
  • Minimums for transitive dependencies are in the new oldestdeps-constraints.txt, applied through UV_CONSTRAINT. Each is the first release after Python 3.12.0 came out (the rule is in the file's header), worked out across platforms with uv pip compile --universal.
• CI reorganized. The default Python is 3.14 and job names describe roles instead of versions, so future Python bumps only change the tox envs and default_python:
  • Linux runs 3.12 and 3.13 with base dependencies.
  • New: Linux 3.12 with the oldest dependencies. oldestdeps wasn't run in CI before.
  • macOS and Windows run 3.14 (previously 3.12).
  • Linux 3.14 runs all dependencies, remote data and coverage.
  • devdeps moves to its own allowed-to-fail job on Python 3.15 (release candidate, installed by uv), with remote data.
  • The weekly pre-releases job is renamed to match.
• No GUI windows during tests. The root conftest.py forces the Agg matplotlib backend.

Minimum versions

Testing the real minimums showed several didn't work:

• numpy>=1.24 → >=1.26.0: 1.24 and 1.25 have no Python 3.12 builds.
• gwcs (unbounded) → >=0.24.0: with specutils 2.0 and gwcs 0.22, creating a Spectrum without a spectral axis fails with UnitConversionError: 'pix' and '' (dimensionless) are not convertible.
• synphot → >=1.3.post0, the first release with Python 3.12 wheels.
• test extra: pytest>=7.4.0, pytest-astropy>=0.11.0, and photutils>=1.11 to match all. tox is dropped from the test extra: that extra is installed inside each tox environment, where tox isn't needed, and the lowest resolution would install an ancient tox there. A new dev extra (pip install -e ".[dev]") installs the test, docs and all extras together with tox and tox-uv for local development.
• asdf-astropy>=0.8.0 is in the constraints file: older versions serialize units through VOUnit and warn that Angstrom is deprecated, which filterwarnings = error makes fatal.

Test change

test_from_asdf_raises_on_unsupported_transform now uses Scale | Polynomial1D rather than a bare Polynomial1D. With asdf < 4, a GWCS whose transform is a single model with a 1-D bounding box fails schema validation when written. to_asdf always writes a compound shift→polynomial, so users aren't affected.

Not included

• Free-threaded Python job. photutils has no free-threaded wheels and can't be built on free-threaded Python (it uses the limited C API), and the test fixtures need it through SynthImage.add_poisson_noise(). It can be added once photutils supports free threading.

Testing

Run locally with tox:

Environment	Result
py312-test-oldestdeps (numpy 1.26.0, astropy 6.0.0, scipy 1.14.1, matplotlib 3.10.0, specutils 2.0.0, gwcs 0.24.0, asdf 3.3.0, pytest 7.4.0)	passes
py312-test, py313-test, py314-test, codestyle	pass
py314-test-alldeps-cov, remote data	passes (239 passed, 0 skipped)
py314-test-alldeps-predeps	passes
py315-test-devdeps (Python 3.15.0rc2; astropy 8.1.dev, numpy 2.6.dev, scipy 2.0.dev)	passes

linkcheck was not run locally.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.52%. Comparing base (4e330bc) to head (bb5e85d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #324   +/-   ##
=======================================
  Coverage   92.52%   92.52%           
=======================================
  Files          18       18           
  Lines        2341     2341           
=======================================
  Hits         2166     2166           
  Misses        175      175           

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