tox: do not try to use Tox version 4

Change-Id: I07edb29249bb75313f16f4d2273437cc6dc38574
diff --git a/tox.ini b/tox.ini
index 6a95b66..f0c9589 100644
--- a/tox.ini
+++ b/tox.ini
@@ -21,6 +21,7 @@
 requires = pip < 20.3
            virtualenv < 20.0
            setuptools < 50.0.0
+           tox < 4
 # NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
 minversion = 3.2.0