Fix dunder-method positional-only parameter discrepancies in third-party stubs (#14529)

stubs/braintree/braintree/attribute_getter.pyi

@@ -4,4 +4,4 @@ class AttributeGetter:
     def __init__(self, attributes: dict[str, Any] | None = None) -> None: ...
     # This doesn't exist at runtime, but subclasses should define their own fields populated by attributes in __init__
     # Until that's done, keep __getattribute__ to fill in the gaps
-    def __getattribute__(self, name: str) -> Any: ...
+    def __getattribute__(self, name: str, /) -> Any: ...