Fix pylint zuul error due to python-libvirt version

Nov 6th saw the release of a new version of python-libvirt.
tox.ini did not have an upper-constraint set for it,
so we picked up that version, which caused zuul to fail
in pylint.

Adding an upper-constraint to fix the issue.

This allows all reviews in stx/utilities to pass zuul
and be able to merge.

Change-Id: Idcdf37fbfb2f8745aadff9aacba828e8166dbc46
Closes-Bug: 1851707
Signed-off-by: Al Bailey <Al.Bailey@windriver.com>

tox.ini

@@ -7,7 +7,10 @@ sitepackages=False
 stxdir = {toxinidir}/..
 
 [testenv]
-install_command = pip install -U {opts} {packages}
+install_command = pip install \
+    -v -v -v \
+    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/stable/stein/upper-constraints.txt} \
+    -U {opts} {packages}
 setenv =
     VIRTUAL_ENV={envdir}
     OS_STDOUT_CAPTURE=1