Update minimal requirement of six to >= 1.16.0
urllib3-1.26.5 to work properly needs six to be
atleast >= 1.16.0. So to ensure when urllib3 is installed
already installed python3-six is upgraded let's set minimum
requirement of six to >= 1.16.0[1] which contains required methods like
six.ensure_text method needed by urllib3-1.26.5, also urllib3
bundles six-1.16.0[2].
[1] https://github.com/urllib3/urllib3/commit/5b047b
[2] https://github.com/urllib3/urllib3/commit/2698537