Use allowlist_externals

This option whitelist_externals was deprecated in favor of the new
allowlist_externals option and is no longer available in recent
versions of tox.

Change-Id: I1663478fa4cf11f052bd6e772171aab885eec3d5

tox.ini

@@ -9,7 +9,9 @@ basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
-whitelist_externals = find
+allowlist_externals =
+  find
+  rm
 commands =
   find . -type f -name "*.pyc" -delete
   stestr run --slowest {posargs}
@@ -19,9 +21,6 @@ commands = {posargs}
 
 [testenv:docs]
 deps = -r{toxinidir}/doc/requirements.txt
-whitelist_externals =
-  find
-  rm
 commands =
   rm -rf doc/build/html doc/build/doctrees
   find . -type f -name "*.pyc" -delete