[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.

Change-Id: I29beb88dd241af72397bf0dd1fc46c98a229425c

tox.ini

@@ -2,6 +2,10 @@
 minversion = 3.1
 envlist = py37,pep8
 ignore_basepython_conflict = true
+# 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]
 basepython = python3