Annotate known magic-method return types (#9131)

stubs/braintree/braintree/errors.pyi

@@ -9,4 +9,4 @@ class Errors:
     @property
     def deep_errors(self): ...
     def for_object(self, key): ...
-    def __len__(self): ...
+    def __len__(self) -> int: ...

stubs/braintree/braintree/validation_error_collection.pyi

@@ -17,4 +17,4 @@ class ValidationErrorCollection:
     @property
     def size(self): ...
     def __getitem__(self, index): ...
-    def __len__(self): ...
+    def __len__(self) -> int: ...