name that occurs in python 3.9

Author: wyattscarpenter

mypy/main.py

@@ -436,6 +436,7 @@ def add_argument_group(
         if title not in [
             "positional arguments",
             "options",
+            "optional argument" # name in python 3.9
         ]:  # These are built-in names, ignore them.
             if not title[0].isupper():
                 raise ValueError(