make #4083 compatible with py3.6

Author: samuelcolvin

pydantic/generics.py

@@ -31,13 +31,13 @@
 
 Parametrization = Mapping[TypeVarType, Type[Any]]
 
-_generic_types_cache: LimitedDict[Tuple[Type[Any], Union[Any, Tuple[Any, ...]]], Type[BaseModel]] = LimitedDict()
+_generic_types_cache: 'LimitedDict[Tuple[Type[Any], Union[Any, Tuple[Any, ...]]], Type[BaseModel]]' = LimitedDict()
 # _assigned_parameters is a Mapping from parametrized version of generic models to assigned types of parametrizations
 # as captured during construction of the class (not instances).
 # E.g., for generic model `Model[A, B]`, when parametrized model `Model[int, str]` is created,
 # `Model[int, str]`: {A: int, B: str}` will be stored in `_assigned_parameters`.
 # (This information is only otherwise available after creation from the class name string).
-_assigned_parameters: LimitedDict[Type[Any], Parametrization] = LimitedDict()
+_assigned_parameters: 'LimitedDict[Type[Any], Parametrization]' = LimitedDict()
 
 
 class GenericModel(BaseModel):

pydantic/utils.py

@@ -784,6 +784,6 @@ def __setitem__(self, __key: Any, __value: Any) -> None:
                 for key in to_remove:
                     del self[key]
 
-        def __class_getitem__(cls, *args: Any) -> Any:
-            # to avoid errors with 3.7
+        def __class_getitem__(cls, *args: Any) -> Any:  # pragma: no cover
+            # just in case LimitedDict is used in type annotations
             pass