packaging: use "python3" for fedora29 ... and as buildrpm default...
packaging: use "python3" for fedora29 ... and as buildrpm default
Change the buidrpm default. The CentOS targets explicitly use "python", and
changing the default will only influence Fedora 29.
A Python 3 package needs python3 dependencies, so pythonexe (and pythonver) is
used for specifying dependencies. Other OS versions will keep using "python" as
before ... or potentially change to explicit "python2". Fedora 29 packages can
thus also still be built for Python 2 - just not in the docker image that is
updated for Python 3.
