Fixes to argparse stubs (Python 3)

Author: JukkaL

stubs/3.2/argparse.pyi

@@ -1,4 +1,4 @@
-# Stubs for argparse (Python 3.4)
+# Stubs for argparse (Python 3)
 #
 # NOTE: This dynamically typed stub was automatically generated by stubgen.
 
@@ -115,10 +115,11 @@ class _ActionsContainer:
     def register(self, registry_name, value, object): ...
     def set_defaults(self, **kwargs): ...
     def get_default(self, dest): ...
-    def add_argument(
+    def add_argument(self,
         *args: str,
         action: str = None,
         nargs: str = None,
+        const: Any = None,
         default: Any = None,
         type: Any = None,
         choices: Any = None, # TODO: Container?