Should enable CI for other versions of python

[azeey]

We currently run CI for python 3.8. We should enable CI for other versions once python bindings for ignition-math and libsdformat are available for those versions.

[j-rivero]

If we want to release python bindings for different versions of python inside our binary packages, instead of only shipping 3.8 in Focal we would need to change packaging on sdformat/math.

• Use pybuild as buildsystem in the debian/rules file with cmake plugin in that system (here for sdformat and here for ign-math). Best example is probably the rules file in official Debian.
• Add python-all package to build-deps in the same repositories control file.
• Be sure that the python version is injected in the buildsystem, IGN_PYHON_VERSION in the gz-cmake python module should help with this.