This patch removes the use of "six" as the package declares only
python3 support.
It also updates the appdirs lower-constraint to 1.4.0 to support
pip. This version is available on both centos8 and focal.
It also removes the linter related packages from lower-constraints.
Change-Id: I9337f1998749bc40737f2f0e2dcc406b6f3a0ddf