[stable-only] Fix gate

- Pin more deps on py2
- Use [tox]requires to pin virtualenv

Related-Change: Ibe46aecf0f4461be59eb206bfe9063cc1bfff706
Related-Change: If81fc352c8b2b1f03f3fa7b79c56dfcf981ced70
Related-Change: I32a161e555179ca34d306ac37e4097611853e36b
Change-Id: I5962eefd9e3626040eec6add17d92bef9b90c47d

py2-constraints.txt

@@ -74,4 +74,8 @@ gitdb2===2.0.6
 # Projects that are known to have had a final py2-supporting release
 bandit===1.6.2
 python-keystoneclient===3.22.0
+dnspython===1.16.0
+setuptools===44.1.1
+pycadf===2.10.0
+smmap===3.0.5
 PasteDeploy==2.1.1

tox.ini

@@ -2,6 +2,11 @@
 envlist = py37,py27,pep8
 minversion = 2.3.2
 skipsdist = True
+requires =
+  # required to support py27/py36 envs
+  virtualenv<20.22
+  # project-wide requirement; see .zuul.yaml
+  tox<4
 
 [testenv]
 usedevelop = True