[stable-only] Cap virtualenv for py37

py37 test started to fail with latest virtualenv (20.16.4) as it
bundles setuptools 65.3.0, which causes installation problems under
py37. (Note: with py38 the problem does not occur).

This patch sets virtualenv<20.16.4 to unblock the gate as
virtualenv 20.16.3 bundles setuptools 63.4.1, with which py37 gate
works well.

NOTE(stephenfin): Modified to remove the (failing) l-c job.

Conflicts:
  .zuul.yaml
  tox.ini

NOTE(elod.illes): conflict in .zuul.yaml is caused by ussuri job
  template changes; conflict in tox.ini is caused by tox.ini cleanup
  in ussuri: I36ef5b39b48e259b51f2bb03caf3443d8969c1e9

Change-Id: I845dca676e1c1e1b3961e8777098a48a6203833e
(cherry picked from commit 76fd064a7f)

.zuul.yaml

@@ -4,7 +4,6 @@
       - lib-forward-testing
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python3-train-jobs
       - periodic-stable-jobs

tox.ini

@@ -1,6 +1,10 @@
 [tox]
 minversion = 2.0
 envlist = py27,py37,pep8
+# Pin setuptools via virtualenv as latest virtualenv pulls in 65.3.0
+# version of setuptools for py37, which causes installation problems
+# (virtualenv 20.16.3 uses setuptools 63.4.1).
+requires = virtualenv<20.16.4
 
 [testenv]
 install_command = pip install {opts} {packages}