1 files changed, 5 insertions, 0 deletions

diff --git a/tox.ini b/tox.ini
index 86f81ee28..8cc365749 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,11 @@
 [tox]
 envlist = py37,py27,pep8
 minversion = 3.18.0
+requires =
+  # required to support py27/py36 envs
+  virtualenv<20.22
+  # project-wide requirement; see .zuul.yaml
+  tox<4
 
 [pytest]
 addopts = --verbose