See https://github.com/Pylons/webob/commit/dbcd121318f4025919869379350b7d15c6b27484

--- webob-1.8.9/tox.ini.orig
+++ webob-1.8.9/tox.ini
@@ -19,8 +19,10 @@
     py2: python2.7
     py3: python3.5
 
+extras =
+    testing
+
 commands =
-    pip install webob[testing]
     py.test tests --junitxml=pytest-{envname}.xml --cov --cov-report=term-missing {posargs:}
 
 setenv =