Require a comma in the preceding items sub-pattern

Author: bswck

Grammar/python.gram

@@ -1492,7 +1492,7 @@ invalid_class_pattern:
         PyPegen_last_item(a, pattern_ty),
         "positional patterns follow keyword patterns") }
 invalid_mapping_pattern:
-    | '{' (items_pattern ','?)? rest=double_star_pattern ',' items_pattern ','? '}' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+    | '{' (items_pattern ',')? rest=double_star_pattern ',' items_pattern ','? '}' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
         rest,
         "double star pattern must be the last (right-most) subpattern in the mapping pattern") }
 invalid_class_argument_pattern[asdl_pattern_seq*]: