[stable-only] Pin virtualenv and setuptools

Setuptools 58.0 (bundled in virtualenv 20.8) breaks the installation of
decorator 3.4.0. So this patch pins virtualenv to avoid the break.

As the used 'require' feature was introduced in tox in version 3.2 [1],
the required minversion has to be bumped, too.

[1] https://tox.readthedocs.io/en/latest/config.html#conf-requires

Change-Id: Ia80ec828869e3c4f3d80f7fe9898a57788ff2b07

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.0
+minversion = 3.2.0
 skipsdist = True
 skip_missing_interpreters = true
 # python runtimes: https://governance.openstack.org/tc/reference/runtimes/ussuri.html
@@ -7,6 +7,10 @@ envlist = py37,py36,compliance,pep8
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file
 ignore_basepython_conflict=true
+# Pin the virtualenv and therefore the setuptools version used for the env
+# creation. This results in a new tox being installed in .tox/.tox virtualenv
+# and the tox on the host will delegate all the calls to the tox in that env.
+requires = virtualenv<20.8
 
 [testenv]
 basepython = python3