Bump minimum tox version to 4.0

With switch of propose-translation-update nodeset to noble,
tox version was bumped and whitelist_externals is no longer
respected there, which leads to job failure.

Change-Id: I075b4f219501ba63d3cd2fb52f6c40e0a18fd153
Signed-off-by: Dmitriy Rabotyagov <dmitriy.rabotyagov@cleura.com>

tox.ini

@@ -1,13 +1,15 @@
 [tox]
-minversion = 3.1.1
+minversion = 4.0
 envlist = publishdocs
 skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
-setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/test-requirements.txt
+setenv =
+  VIRTUAL_ENV={envdir}
+deps =
+  -r{toxinidir}/test-requirements.txt
 allowlist_externals =
   mkdir
   rm
@@ -46,7 +48,8 @@ allowlist_externals =
   {toxinidir}/tools/add-marker.sh
   {toxinidir}/tools/buildlang.sh
 # These are needed for writing the root-marker
-passenv = ZUUL*
+passenv =
+  ZUUL*
 commands =
   {[testenv:upstream-slides]commands}
   # Publish