Recognize dictionary unpacking operation

Author: Thirumalai-Shaktivel

src/lpython/parser/parser.yy

@@ -782,6 +782,7 @@ expr_list
 dict
     : expr ":" expr { $$ = DICT_EXPR_01($1, $3, @$); }
     | expr ":" type_comment expr { $$ = DICT_EXPR_02($1, $3, $4, @$); }
+    | "**" expr { $$ = DICT_EXPR_01(NONE(@$), $2, @$); }
     ;
 
 dict_list

src/runtime/lpython_parser.py

@@ -78,6 +78,8 @@ def generic_visit(self, node):
                     else:
                         if type(item) == str:
                             new_list.append(item)
+                        elif item is None:
+                            new_list.append(self.visit(python_ast.ConstantNone()))
                 d[field] = new_list
             elif field in ["vararg", "kwarg"]:
                 if value is None: