Simplify macro.pythonpath.eggs and add a test in Jupyter to check that numpy can really be imported.

Background:

Macro macro.pythonpath.eggs was originally introduced to inject PYTHONPATH into a section holding environment variables for zc.recipe.egg:custom in order to make the cython egg available as a setup dependency for numpy (cython is needed to run numpy's setup.py).

Recently we looked at it with @lpgeneau, who has a similar use case to inject PYTHONPATH in slapos.recipe.cmmi environment. And we realised that macro.pythonpath.eggs was needlessly complicated and could be much simpler (and therefore understandable and maintainable). So we changed it, and to make sure this change didn't break numpy I wrote a jupyter test that actually imports numpy.