Fix another mention of whitelist_externals in tox.ini

Author: ldionne

tox.ini

@@ -52,7 +52,7 @@ commands =
 [testenv:runserver]
 # Don't use this for production. Just a handy target
 # for local testing.
-whitelist_externals = sh
+allowlist_externals = sh
 commands =
     sh -c "if [[ ! -e ./server_sandbox/ ]]; then lnt create ./server_sandbox/; fi"