ci: remove Python package build dependencies for Python 3.14

[jtligon]

Summary

Removes the system build dependencies (libjpeg-dev, gfortran, libopenblas-dev, etc.) added for Python 3.14, as pre-built wheels are now available for both Pillow and scipy.

Background

When Python 3.14 support was added in #775, neither Pillow nor scipy had pre-built wheels yet, so we needed to install system libraries to build them from source.

When to merge

DO NOT MERGE until both Pillow and scipy publish Python 3.14 wheels to PyPI.

How to verify wheels are available:

1. Check PyPI:

   • Pillow: https://pypi.org/project/pillow/#files (look for cp314 wheels)
   • scipy: https://pypi.org/project/scipy/#files (look for cp314 wheels)

2. Check CI logs (easier):

   • Currently shows: Building pillow==11.2.1 and Building scipy==1.16.0
   • After wheels available: Downloaded pillow and Downloaded scipy

CI signals to watch:

Run a test CI build and check the pytest-matrix logs:

# Currently (building from source):
Building pillow==11.2.1
Building scipy==1.16.0

# After wheels available (ready to merge):
Downloaded pillow
Downloaded scipy

Impact

• Before: ~10-20s overhead per Python 3.14 CI job for unused package installation
• After: No overhead, faster CI

Closes

#795

Checklist before merging

• Pillow has published cp314 wheels to PyPI
• scipy has published cp314 wheels to PyPI
• CI logs show Downloaded pillow and Downloaded scipy instead of building
• Python 3.14 pytest jobs still pass without build dependencies

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