Fix docs build for tox4

Replace deprecated whitelist_externals with allowlist_externals. Drop
skipsdist = True setting.

Change-Id: Iced0ce2472e5c0bd09fba0d274ef2772883a0a49

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 3.1.1
-skipsdist = True
-envlist = py39,pep8,releasenotes
+envlist = py3,pep8,releasenotes
 ignore_basepython_conflict = True
 
 [testenv]
@@ -14,7 +13,7 @@ deps =
     -r{toxinidir}/test-requirements.txt
     -r{toxinidir}/requirements.txt
 commands = stestr run {posargs}
-whitelist_externals =
+allowlist_externals =
   bash
 basepython = python3
 
@@ -85,7 +84,7 @@ setenv =
     SPHINX_APIDOC_OPTIONS=noindex,members,undoc-members,show-inheritance
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
   rm
 commands =