[stable-only] Pin tox <4stable/xena

Continue to use tox 3 in the stable branches.

Two changes:
- .zuul.yaml: set the ensure_tox_version to use <4, which will
  tell zuul to install tox<4 if it's not present
- tox.ini: set requires=tox<4 so that if tox has been installed
  already, our tox-based jobs will use tox 3 to run the tests

Change-Id: Ica4c0a1d4f861e528ce8995766e82541dc710e0f
(cherry picked from commit 22a3169576b3fd91388e7b57ffd39e3760016796)
(cherry picked from commit e3799da5558a6a686885fa70c155d8d37d44497d)

2 files changed, 3 insertions, 0 deletions

diff --git a/.zuul.yaml b/.zuul.yaml
index 28853f4..5342339 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -39,6 +39,8 @@
       tox_envlist: functional-py38
 
 - project:
+    vars:
+      ensure_tox_version: '<4'
     templates:
       - check-requirements
       - lib-forward-testing-python3
diff --git a/tox.ini b/tox.ini
index a19c6b7..818f967 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,6 +2,7 @@
 distribute = False
 envlist = py3,pep8
 minversion = 3.18.0
+requires = tox<4
 skipsdist = True
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file