Pin tox to be less than 4

The dev environment used by some docs developers uses
an old version of tox, and does not support the
allowlist_externals entry required by tox 4.

The version of tox used by some other developers cannot
support tox.ini that contains both allowlist_externals and
whitelist_externals in the same file.

The fix for now is to stay with whitelist_externals and tox<4

This fix mostly reverts and re-works these two commits
https://review.opendev.org/c/starlingx/docs/+/869229
https://review.opendev.org/c/starlingx/docs/+/868621

Partial-Bug: #2000399

Signed-off-by: Al Bailey <al.bailey@windriver.com>
Change-Id: Ib8a5eea5d27e7b7347bc717d11f9dc5a0be4f396

.zuul.yaml

@@ -1,5 +1,7 @@
 ---
 - project:
+    vars:
+      ensure_tox_version: '<4'
     templates:
       - stx-api-ref-jobs
     check:

tox.ini

@@ -29,24 +29,24 @@ commands =
   git restore doc/source/dist_cloud/kubernetes/*
   bash hw-updates.sh
   bash htmlChecks.sh doc/build/html
-allowlist_externals = bash
+
+# Note: The dev env for the docs team uses a version of tox that does not yet support allowlist_externals
+whitelist_externals = bash
                       htmlChecks.sh
                       get-remote-files.sh
                       git
 #                      hw-updates.sh
-whitelist_externals = {[testenv:docs]allowlist_externals}
+
 
 [testenv:api-ref]
 deps = {[testenv:docs]deps}
 commands =
   rm -rf api-ref/build
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
-allowlist_externals = rm
+whitelist_externals = rm
-whitelist_externals = {[testenv:api-ref]allowlist_externals}
 
 [testenv:linters]
-allowlist_externals = bash
+whitelist_externals = bash
-whitelist_externals = {[testenv:linters]allowlist_externals}
 commands =
   bash -c "find {toxinidir}                   \
             \( -name .tox -prune \)           \
@@ -64,16 +64,14 @@ passenv=PWD
 commands =
        bash -c "echo Running in {env:PWD}"
        bash new-topic.sh {env:PWD} {toxinidir}
-allowlist_externals = new-topic.sh
+whitelist_externals = new-topic.sh
                       bash
-whitelist_externals = {[testenv:newfile]allowlist_externals}
 
 [testenv:picks]
 commands =
        ./pickCompare.sh
-allowlist_externals = pickCompare.sh
+whitelist_externals = pickCompare.sh
                       bash
-whitelist_externals = {[testenv:picks]allowlist_externals}
 
 
 [testenv:linkcheck]