Yes and yes:
Ad python3_sitelib - build otherwise fails while trying to find out python2_sitelib stuff, regardless of skip_python2 being defined - it might be bug of python-rpm-macros on SLE-12
Ad SLE-12 - of course we support them, reason why I am doing this excesise is that we need this package build for SLE-12 as dependency of avocado which is requirement from QAM
Are you certain that the %{python3_sitelib} is really truly necessary on SLE12? Do we need to support SLE12?
Yes and yes: Ad python3_sitelib - build otherwise fails while trying to find out python2_sitelib stuff, regardless of skip_python2 being defined - it might be bug of python-rpm-macros on SLE-12
Ad SLE-12 - of course we support them, reason why I am doing this excesise is that we need this package build for SLE-12 as dependency of avocado which is requirement from QAM