demos: cppextensions Makefile: python may not be present in PATH on generic systems

The doc/demos/cppextensions/Makefile file now call python in a shell command. On generic Debian systems, python does not exist, but python3 does and python2 might be inherited from old-stable Debian distributions. Debian does not recommend to install python. I have no idea what is the policy on other systems regarding python, python2, and python3. Does the python in the Makefile meant to be either python2 or python3 ? In other words, does graph-tool support both python 2 and python 3 ? If graph-tool no more support python 2, I would suggest to call in the Makefile python3 instead. Note that python3 is actually in Debian a link to a python 3 dot something. This looks as a very reasonable choice that we may find in most distributions and on most systems.