ast: add visit_Constant to NodeVisitor (#3767)

stdlib/3/ast.pyi

@@ -19,6 +19,8 @@ else:
 class NodeVisitor:
     def visit(self, node: AST) -> Any: ...
     def generic_visit(self, node: AST) -> Any: ...
+    if sys.version_info >= (3, 8):
+        def visit_Constant(self, node: Constant) -> Any: ...
 
 class NodeTransformer(NodeVisitor):
     def generic_visit(self, node: AST) -> Optional[AST]: ...