dynamically test Python versions based on Python version pins in metadata

[zacharyburnett]

Currently, the Python versions defined in our CI workflows are defined by passing a static list of envs to .github/workflows/tox.yml. These need to be manually updated when 1) new Python versions are released as latest, and 2) we drop support for old versions of Python.

Possibly as an alternative to envs:, we could add a flag (supported_pythons?) that does the following:

1. retrieve the oldest and latest major versions of Python supported by the project, based on pins in pyproject.toml project.requires-python
   • to start, this only needs to consider the metadata of the top-level project, but perhaps we could also scan dependencies?
2. build a matrix on-the-fly of each major version of Python between the oldest and latest. For instance, if the oldest supported version of the package is Python 3.11, and the latest is Python 3.14, then the test matrix would be 3.11, 3.12, 3.13, 3.14

[Cadair]

Wouldn't a more "tox-first" solution to this be to have an option to run all the jobs in envlist?

$ tox list
default environments:
py311                -> run tests
py312                -> run tests
py313                -> run tests
py313-online         -> run tests that require remote data (as well as the offline ones)
py313-minimal        -> run tests
py313-devdeps        -> run tests with the latest developer version of key dependencies
py311-oldestdeps     -> run tests with the oldest supported version of key dependencies
py312-figure         -> run tests runs the figure test suite.
py312-figure-devdeps -> run tests with the latest developer version of key dependencies runs the figure test suite.
build_docs           -> Invoke sphinx-build to build the HTML docs
build_docs-gallery   -> Invoke sphinx-build to build the HTML docs
codestyle            -> Run all style and file checks with pre-commit
core_deps            -> Check the test suite does not fail if only the core dependencies are installed.
linkcheck            -> Invoke sphinx-build to check the URLS within the docs

additional environments:
coordinates          -> Run sunpy.coordinates specific tests with full functionality
conda                -> run tests with the latest conda version of key dependencies

Then at least you would only need to update tox.ini when changing python support?

[zacharyburnett]

that's also a good idea, we can do that too, but keeping envlist updated across all our pipelines might be too much so I'd prefer to have a way to generate the envs dynamically as well. Looking back on this though, this should probably be an extra script that spits out a YAML block, not necessarily part of the tox.yml workflow itself. I'll play around with it

[astrofrog]

For the record I would quite like the ability to just do py3*-test (maybe a wildcard makes sense rather than just py3?)