Ammend annotation for __slots__

This fixes an assignablity issue in subclasses that was caused by using too strict an annotation for the purpose of `__slots__` as a special case for mypy.

We can keep an annotation here for mypy if really needed, but it needs to work with subclass assignability.

ideally, long term all data model names are understood as part of the specification, but this needs to be permissive enough to allow subclasses to add members.

See error in pyright as a result of adding a special case for mypy: microsoft/pyright#11058

Author: mikeshardmind

stdlib/asyncio/protocols.pyi

@@ -14,7 +14,7 @@ class BaseProtocol:
 
 class Protocol(BaseProtocol):
     # Need annotation or mypy will complain about 'Cannot determine type of "__slots__" in base class'
-    __slots__: tuple[()] = ()
+    __slots__: tuple[str, ...] = ()
     def data_received(self, data: bytes) -> None: ...
     def eof_received(self) -> bool | None: ...