Blender Git Loki

Installdeps: better handling of python versions.

now that we stick to some outdated py version, some distro (like current
debian testing) will feature several python3 dev package, but other
dependant libs like numpy are only built against current default version
of python (3.8 now in deb testing)...

In order to be able to use distro packages we need to allow using higher
versions of python, and set relevant CMake option accordingly.