GHA: MacOS failure to import numpy

[KrisThielemans]

https://github.com/UCL/STIR/runs/4716700749?check_suite_focus=true#step:11:37

/usr/local/Cellar/python@3.9/3.9.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
swig/test/python/test_IO.py:14: in <module>
    from stir import *
../install/python/stir.py:15: in <module>
    from _stir import *
E   ImportError: numpy.core.multiarray failed to import
------------------------------- Captured stderr --------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/numpy/__init__.py", line 380, in <module>
    raise RuntimeError(msg)

More than likely due to version conflict between the brew Python and another one, see e.g. https://stackoverflow.com/questions/58342005/macos-python3-with-brew-error-on-import-cv2

[KrisThielemans]

#985 reorganised things, which is good, but did not fix the actual pytest failure, so re-opening. See there for more comments.

#985 disabled the pytest on MacOS until someone comes around to fix this. Presumable it'll be fixed by numpy, GHA or whoever (but not me)