Don't build the python3 package on RHEL
    
    Also separates the decision of building with python3 from the decision
    of defining python_siteach manually, using the Python packaging
    guidelines condition for the latter.
    
    I'll build this for F18 as well without submitting as an update - the
    Fedora users don't need this change, but it keep the Fedora and RHEL
    versioning consistent for a while longer.  This will hopefully make
    maintenance a bit easier, and doesn't harm anybody.