Revert "python --> python3" (DO NOT MERGE)

[fingolfin]

This reverts commit 062043b.

I want to see if this fixes CI, and if so, compare the logs.

[w-bruns]

There are 3 warnings about differing MacOS versions. The first is

ld: warning: dylib (/Users/runner/work/PyNormaliz/PyNormaliz/local/lib/libnormaliz.dylib) was built for newer macOS version (12.0) than being linked (11.7)

I simply don't understand what it could mean. We ask for MacOS:latest which I think is 12.0. So libnormaliz.dylib should be built for it in this run. But how can it be linked for 11.7? Are there old components in the system?

It is of course unclear whether the warnings have to do anything with the failure. Unfortunately the last successful run from February has lost its logs.

In the Linux build on my laptop under Ubuntu I also see

zip_safe flag not set; analyzing archive contents...
__pycache__.PyNormaliz_cpp.cpython-310: module references __file__

But then

creating 'dist/PyNormaliz-2.18-py3.10-linux-x86_64.egg' and adding 'build/bdist.linux-x86_64/egg' to it

and this (or a similar) message is not created in the MacOS build on GitHub.