[CI][stable-only] Workaround for missing pkg_resources

With recent virtualenv release (that bundles setuptools in it) tox jobs
started to fail with 'missing pkg_resources module' errors.
Since this is an old branch, where we have only python version <= 3.11
in gate jobs, it is enough to cap virtualenv to fix the gate.

Change-Id: If10ce13899edaf6dc1d5798aee17842e54939bc7
Signed-off-by: Elod Illes <elod.illes@est.tech>
(cherry picked from commit dd8c061)

Author: Elod Illes

tox.ini

@@ -1,6 +1,12 @@
 [tox]
 minversion = 3.18.0
 envlist = py3,functional,pep8
+# NOTE(elod.illes): latest virtualenv bundles setuptools 82.0.0, which
+# dropped pkg_resources module source from the package, which is used
+# in nova and other packages, thus virtualenv needs to be pinned to fix
+# the gate.
+requires =
+    virtualenv<20.37.0
 
 [testenv]
 usedevelop = True