tox.ini: Replace whitelist_externals by allowlist_externals

Was replaced upstream, see tox-dev/tox#2600

Author: hartwork

tox.ini

@@ -59,7 +59,7 @@ commands =
       {toxinidir}
     # metadata validation
     sh -c "python -m twine check {toxinidir}/dist/*"
-whitelist_externals =
+allowlist_externals =
     rm
     sh