TypeVisitor and Co now support allow_interpreted_subclasses=True (#9602)

Now these types can be extended from plugin code.
More context: #9001 (comment)

Author: sobolevn

mypy/type_visitor.py

@@ -14,7 +14,7 @@
 from abc import abstractmethod
 from mypy.ordered_dict import OrderedDict
 from typing import Generic, TypeVar, cast, Any, List, Callable, Iterable, Optional, Set, Sequence
-from mypy_extensions import trait
+from mypy_extensions import trait, mypyc_attr
 
 T = TypeVar('T')
 
@@ -28,6 +28,7 @@
 
 
 @trait
+@mypyc_attr(allow_interpreted_subclasses=True)
 class TypeVisitor(Generic[T]):
     """Visitor class for types (Type subclasses).
 
@@ -104,6 +105,7 @@ def visit_type_alias_type(self, t: TypeAliasType) -> T:
 
 
 @trait
+@mypyc_attr(allow_interpreted_subclasses=True)
 class SyntheticTypeVisitor(TypeVisitor[T]):
     """A TypeVisitor that also knows how to visit synthetic AST constructs.
 
@@ -134,6 +136,7 @@ def visit_placeholder_type(self, t: PlaceholderType) -> T:
         pass
 
 
+@mypyc_attr(allow_interpreted_subclasses=True)
 class TypeTranslator(TypeVisitor[Type]):
     """Identity type transformation.
 
@@ -241,6 +244,7 @@ def visit_type_alias_type(self, t: TypeAliasType) -> Type:
         pass
 
 
+@mypyc_attr(allow_interpreted_subclasses=True)
 class TypeQuery(SyntheticTypeVisitor[T]):
     """Visitor for performing queries of types.