BLD: use gfortran 15 for f2py tests on macOS

[fxcoudert]

I saw that you recently updated the macOS Intel runner, I would like to suggest that you also update the gfortran version used for compilation. There have been some improvements in macOS support (on both Intel and arm64).

[rgommers]

Thanks @fxcoudert. Note that gfortran isn't used at all for building, only for testing f2py - so this PR doesn't really change anything. I'll run CI anyway to check, but it should not matter.

[fxcoudert]

I got confused then, because I think the wheels do ship libgfortran. On my machine I have:

$ ls venv/lib/python3.11/site-packages/numpy.libs 
libgfortran-040039e1-0352e75f.so.5.0.0*  libquadmath-96973f99-934c22de.so.0.0.0*  libscipy_openblas64_-8fb3d286.so*

And also here:

numpy-release/tools/wheels/LICENSE_osx.txt

Line 99 in e6d5e96

Files: numpy/.dylibs/libgfortran*, numpy/.dylibs/libgcc*

And in the generated wheel for macOS ARM with openblas:

$ ls numpy/.dylibs 
libgcc_s.1.1.dylib         libgfortran.5.dylib        libquadmath.0.dylib        libscipy_openblas64_.dylib

[rgommers]

Yes, they're pulled in by the vendoring process (delocate/auditwheel/delvewheel) when we are linking against scipy-openblas64. If that libgfortran needs updating, the source repo for it is at https://github.com/MacPython/openblas-libs.

[rgommers]

CI is green, so I'm just going to merge this - may as well. Thanks @fxcoudert