CI: scipy installation failing in umfpack tests

[dschmitz89]

Describe your issue.

See a recent CI log: https://github.com/scipy/scipy/actions/runs/9085915867/job/24970362158?pr=20713

Reproducing Code Example

n/a

Error message

n/a

SciPy/NumPy/Python version and system information

n/a

[tylerjereddy]

The conda/mamba dependency resolver seems to be using Python 3.12 while that CI matrix entry intentionally specifies python-version: ["3.11"]. We do use the GHA script conda-incubator/setup-miniconda@v3 with python-version: ${{ matrix.python-version }}, but perhaps the unpinned python version in environment.yml is starting to cause issues via mamba env update -n scipy-dev -f environment.yml. @h-vetinari is that plausible?

[h-vetinari]

@h-vetinari is that plausible?

Yes. It needs to be pinned to 3.11 once the environment gets created, later updates will not change the python version in the environment unless explicitly asked (and since scikit-umfpack isn't yet available for 3.12, it fails).