stubtest: enable checking pos-only differences in dunders (#12184)

Author: AlexWaygood

mypy/stubtest.py

@@ -594,7 +594,6 @@ def _verify_signature(
             runtime_arg.kind == inspect.Parameter.POSITIONAL_ONLY
             and not stub_arg.variable.name.startswith("__")
             and not stub_arg.variable.name.strip("_") == "self"
-            and not is_dunder(function_name, exclude_special=True)  # noisy for dunder methods
         ):
             yield (
                 'stub argument "{}" should be positional-only '