fix __metaclass_ references (#2402)

third_party/2and3/google/protobuf/descriptor.pyi

@@ -8,8 +8,7 @@ class TypeTransformationError(Error): ...
 class DescriptorMetaclass(type):
     def __instancecheck__(cls, obj): ...
 
-class DescriptorBase:
-    __metaclass__ = DescriptorMetaclass
+class DescriptorBase(metaclass=DescriptorMetaclass):
     has_options = ...  # type: Any
     def __init__(self, options, options_class_name) -> None: ...
     def GetOptions(self): ...

third_party/2and3/yaml/__init__.pyi

@@ -36,8 +36,7 @@ def add_multi_representer(data_type, multi_representer, Dumper=...): ...
 class YAMLObjectMetaclass(type):
     def __init__(cls, name, bases, kwds) -> None: ...
 
-class YAMLObject:
-    __metaclass__ = YAMLObjectMetaclass
+class YAMLObject(metaclass=YAMLObjectMetaclass):
     yaml_loader = ...  # type: Any
     yaml_dumper = ...  # type: Any
     yaml_tag = ...  # type: Any