installPackagesRunTests.sh: Try higher python versions

The install installPackagesRunTests.sh is still useful for people who use virtualenv (in some version) to isolate the different python environment. Tested and used on macos with mac ports. It turns out that python is going to have new versions every now and then, and it does not make sens to name them all in the script. Refactor it to use "all" python 3.n versions, where n goes from 19 (which does not yet exist) down to 3.6, which is the lowest usable version.