Pin tox to 3

There are many issues with toxv4 that make it difficult to use with
Zuul.  Pin to tox version 3 while we find a solution.

Change-Id: I608b2ad4ab9407d8a0b77d5def5188922875e00f

.zuul.yaml

@@ -278,6 +278,7 @@
     vars:
       node_version: 16
       release_python: python3
+      ensure_tox_version: "<4"
     check:
       jobs:
         - zuul-build-image