Make zerver/lib/actions.py pass mypy check.

tools/run-mypy

@@ -19,7 +19,6 @@ docs/conf.py
 puppet/zulip_internal/files/postgresql/pg_backup_and_purge.py
 tools/deprecated/
 zproject/
-zerver/lib/actions.py
 zerver/lib/bugdown/fenced_code.py
 zerver/lib/statistics.py
 zerver/middleware.py

zerver/lib/actions.py

@@ -786,7 +786,7 @@ def extract_recipients(s):
         data = s
 
     if isinstance(data, six.string_types):
-        data = data.split(',')
+        data = data.split(',') # type: ignore # https://github.com/python/typeshed/pull/138
 
     if not isinstance(data, list):
         raise ValueError("Invalid data type for recipients")