Use FindPython, and improve GHA to use venv

[KrisThielemans]

Fixes #979
(but not just yet)

[KrisThielemans]

I think the underlying problem was actually that CMake found a different Python library from the one that was used at run-time. I've therefore now switched to FindPython. But now I get a different error (sigh)

  File "/usr/local/lib/python3.9/site-packages/numpy/__init__.py", line 380, in <module>
    raise RuntimeError(msg)
RuntimeError: Polyfit sanity test emitted a warning, most likely due to using a buggy Accelerate backend.
If you compiled yourself, more information is available at:
https://numpy.org/doc/stable/user/building.html#accelerated-blas-lapack-libraries
Otherwise report this to the vendor that provided NumPy.

Some related info on this error:
https://trac-test.macports.org/ticket/61490 and
numpy/numpy#20418. Those say that the error went away, but here it is again...

[KrisThielemans]

numpy/numpy#15947 (comment) says to rebuild numpy make sure it doesn't use Accelerate but OpenBLAS. oh well

[KrisThielemans]

I can't rebuild numpy. I tried

python3 -m pip install --no-cache-dir --no-binary numpy numpy

and now get

RuntimeError: Setuptools version is '60.5.0', version < '60.0.0' is required. See pyproject.toml

@robbietuk maybe you can give some advice on how turn on MacOS these days

[KrisThielemans]

the latter is mentioned at numpy/numpy#20824

[robbietuk]

As mentioned here: (numpy/numpy#20824 (comment)), you may want to try a venv?

[KrisThielemans]

so... i think I have finally created the virtualenv correctly. python -c 'import numpy; print(numpy.pi); import numpy.core.multiarray works, but pytest still fails with the Polyfit sanity test. https://github.com/UCL/STIR/runs/4977297464?check_suite_focus=true#step:11:74

Looks like I have to try with rebuilding numpy anyway.

madness

[KrisThielemans]

rebuilding numpy didn't help

[KrisThielemans]

I've cleaned-up the history and squashed all the GHA commits.

[KrisThielemans]

I tried one more time with MacOS with some extra printout of Python libraries

.... Python_LIBRARIES: /usr/local/opt/python@3.9/Frameworks/Python.framework/Versions/3.9/lib/libpython3.9.dylib
.... Python_INCLUDE_DIRS: /usr/local/opt/python@3.9/Frameworks/Python.framework/Versions/3.9/include/python3.9
.... Python_NumPy_INCLUDE_DIRS: /Users/runner/work/STIR/STIR/my-env/lib/python3.9/site-packages/numpy/core/include

This looks all perfectly fine, So I have no idea what's going on.

I'll disable the MacOS Python check and merge.