Nail pip 20.2.3

Co-authored-by: Alex Kavanagh <alex.kavanagh@canonical.com>

pip.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+
+pip install pip==20.2.3
+pip install "$@"

tox.ini

@@ -22,7 +22,7 @@ minversion = 3.2.0
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 install_command =
-  pip install {opts} {packages}
+  {toxinidir}/pip.sh install {opts} {packages}
 
 commands = nosetests --with-coverage --cover-package=zaza.openstack {posargs} {toxinidir}/unit_tests