python: do not add main brewed Python site-package to virtualenv

[iMichka]

If added, this makes the virtualenv read the main site-package from brewed Python,
and especially makes it read our sitecustomize.py file, which will
modify the sys.executable path.

See the full discussion at:
#8873

I also took the opportunity to not include test deps, as these will
be not be installed, so the .pth file should not contains references
to site-packages from test deps.

Previous packages on Linux did already contain the wrong lines in the pth file,
for example:
cat /home/linuxbrew/.linuxbrew/Cellar/aws-google-auth/0.0.36_1/libexec/lib/python3.8/site-packages/homebrew_deps.pth
import site; site.addsitedir('/home/linuxbrew/.linuxbrew/opt/python@3.8/lib/python3.8/site-packages')
import site; site.addsitedir('/home/linuxbrew/.linuxbrew/opt/libxml2/lib/python3.8/site-packages')

This might have caused subtle bugs for some packages but not for others.

• Have you followed the guidelines in our Contributing document?
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• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

---

[iMichka]

On Mac, dep_site_packages points to: /usr/local/opt/python@3.9/lib/python3.9/site-packages (for the python 3.9 dependency), which does not exist.

On Linux, dep_site_packages points to: /home/linuxbrew/.linuxbrew/opt/python@3.9/lib/python3.9/site-packages (for the python 3.9 dependency), which does exist.

This is why there was a .pth for Linux and not for Mac. The filtering was done through the next unless dep_site_packages.exist?.

Nevertheless, I do believe this patch is a good thing and another level of security for our virtual env.

[iMichka]

dep_site_packages should have been /usr/local/opt/python@3.9/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages on Mac, and not /usr/local/opt/python@3.9/lib/python3.9/site-packages. If that would have been the case, the bug would have also been there on Mac.

Actually there were 2 bugs here. Once this is merged, I might check the dep_site_packages variable on Mac but that's not urgent right now.