Fix for _set_session_key as well

Author: ljodal

django-stubs/contrib/sessions/backends/base.pyi

@@ -25,7 +25,7 @@ class SessionBase(Dict[str, Any]):
     def clear(self) -> None: ...
     def is_empty(self) -> bool: ...
     def _get_session_key(self) -> Optional[str]: ...
-    def _set_session_key(self, value: str) -> None: ...
+    def _set_session_key(self, value: Optional[str]) -> None: ...
     @property
     def session_key(self) -> Optional[str]: ...
     @property