Start to deprecate Python 2 and 3.5
    
    Python 2 will reach EOL in 18 months. Start to issue deprecation
    warnings on Python 2.
    
    No longer claim support for Python 3.5. Python 3.5 is untested.
    
    NOTE: At first I tried to raise the deprecation warning from
    ipalib.__init__. This caused some unforseen side-effects with
    ipaplatform namespace package on Python 2. Eventually it was easier to
    raise the deprecation warning in ipaplatform. RHEL and Debian platforms
    don't raise the deprecation warning yet, because they use Python 2.
    
    Fixes: https://pagure.io/freeipa/issue/7568
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Tibor Dudlak <tdudlak@redhat.com>