virtualenv version pin is too strict.

--- webob-1.8.9/tox.ini.orig
+++ webob-1.8.9/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-requires = virtualenv<20.22.0
+requires = virtualenv
 envlist =
     py27,py34,py35,py36,py37,py313,pypy,
     docs,coverage,pep8