spkg-configure.m4 for pythons

[dimpase]

do not install pythonX if we are in the virtenv of pythonX.

At the moment I don't know how feasible it is - one small obstacle might be that we really do not want to mess with two pythons at the same time.

while it should be possible/desirable to use a system python, any additional Python packages should optionally be installable into a virtualenv instead of the system site-packages, including sagelib itself unless the system happens to meet all of sagelib's Python dependencies already. Listing some relevant Python packages below:

• cysignals
• cython
• ipython
• jinja2
• jupyter_core
• numpy
• scipy
• sympy
• pip
• setuptools
• pkgconfig
• six

[embray]

As the Sage-8.8 release milestone is pending, we should delete the sage-8.8 milestone for tickets that are not actively being worked on or that still require significant work to move forward. If you feel that this ticket should be included in the next Sage release at the soonest please set its milestone to the next release milestone (sage-8.9).

[embray]

For Python packages when installing using the system Python we definitely need a well thought out approach, likely with different options for handling questions such as where to install additional Python packages.

$SAGE_LOCAL already acts as a sort-of virtualenv, but we might still consider using virtualenv/venv in $SAGE_LOCAL (with the --system-site-packages option) as a way of extending sys.path while still using as many system Python packages as possible.