tox: Replace whitelist_externals

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

Change-Id: Ica83dee466006e2753b0231753b89d4bb920e9c4

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py3,pep8
 ignore_basepython_conflict = true
 
@@ -35,7 +35,7 @@ commands =
     coverage report
 
 [testenv:docs]
-whitelist_externals =
+allowlist_externals =
     rm
 deps =
     -r{toxinidir}/doc/requirements.txt
@@ -53,7 +53,7 @@ ignore = E123,E125,E731,W504
 exclude = .venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 
 [testenv:releasenotes]
-whitelist_externals =
+allowlist_externals =
     rm
 deps = {[testenv:docs]deps}
 commands =