fix annotation for BaseCommand.handle() (#201)

Author: mkurnikov

django-stubs/core/management/base.pyi

@@ -63,7 +63,7 @@ class BaseCommand:
         fail_level: int = ...,
     ) -> None: ...
     def check_migrations(self) -> None: ...
-    def handle(self, *args: Any, **options: Any) -> None: ...
+    def handle(self, *args: Any, **options: Any) -> Optional[str]: ...
 
 class AppCommand(BaseCommand):
     missing_args_message: str = ...