Pin tox<4.0.0 for <=stable/zed branch testing

Tox 4.0.0 has some incompatible changes, epecially more
strict on allowlist_externals. Tempest recently changed
allowlist_externals not to be *[1] causing the failure
on jobs where lib/tempest failing to run the tempest
as command in virtual env.

----------
venv: commands[0]> tempest verify-config -uro /tmp/tmp.qH5KgJHTF4
venv: failed with tempest is not allowed, use allowlist_externals to allow it
------

We do not need to test/fix the <=stable/zed branches with tox 4.0.0
and pinning them with the compatible tox version of the time stable
brnaches were releaased is better way.

This commit proposes:
1. Pinning the tox<4.0.0 for <=stable/ze branches testing
2. Workaround to unblock the master gate by pinning it <4.0.0 but
we should make our testing compatible with tox 4.0.0 soon.

Depends-On: https://review.opendev.org/c/openstack/devstack/+/867066

Related-Bug: #1999183

[1] https://review.opendev.org/c/openstack/tempest/+/865314 devstack based job started failing to run tempest command on venv.

Change-Id: I9a138af94dedc0d8ce5a0d519d75779415d3c30b

Author: Ghanshyam Mann

lib/tempest

@@ -779,7 +779,12 @@ function configure_tempest {
 # install_tempest() - Collect source and prepare
 function install_tempest {
     git_clone $TEMPEST_REPO $TEMPEST_DIR $TEMPEST_BRANCH
-    pip_install 'tox!=2.8.0'
+    # NOTE(gmann): Pinning tox<4.0.0 for stable/zed and lower. Tox 4.0.0
+    # released after zed was released and has some incompatible changes
+    # and it is ok not to fix the issues caused by tox 4.0.0 in stable
+    # beanches jobs. We can continue testing the stable/zed and lower
+    # branches with tox<4.0.0
+    pip_install 'tox!=2.8.0,<4.0.0'
     pushd $TEMPEST_DIR
     # NOTE(gmann): checkout the TEMPEST_BRANCH in case TEMPEST_BRANCH
     # is tag name not master. git_clone would not checkout tag because

playbooks/tox/pre.yaml

@@ -5,4 +5,10 @@
       bindep_profile: test
       bindep_dir: "{{ zuul_work_dir }}"
     - test-setup
-    - ensure-tox
+    # NOTE(gmann): Pinning tox<4.0.0 for stable/zed and lower. Tox 4.0.0
+    # released after zed was released and has some incompatible changes
+    # and it is ok not to fix the issues caused by tox 4.0.0 in stable
+    # beanches jobs. We can continue testing the stable/zed and lower
+    # branches with tox<4.0.0
+    - role: ensure-tox
+      ensure_tox_version: "<4"