1 files changed, 2 insertions, 0 deletions
diff --git a/tox.ini b/tox.ini
index 84b00d64..334b8d6b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,6 +2,8 @@
 minversion = 3.18.0
 envlist = py3,functional,pep8
 ignore_basepython_conflict=true
+requires =
+  tox<4
 
 [testenv]
 basepython = python3