* Silence Python 3.5 EOL notice in pip. * Skip import test on compat/selinux.py utils. * Improve Python version skip warning. * Use Python 3.6 as minimum Python for sanity tests. * Improve min Python handling for code-smell tests. * Overhaul test-constraints sanity test. * Merge sanity test constraints with requirements. * Remove legacy content specific constraints. * Add changelog fragment.