lastgenre: Hint mypy to Album.items()

instead of obj.items()

Author: JOJ0

beetsplug/lastgenre/__init__.py

@@ -436,6 +436,7 @@ def _try_resolve_stage(stage_label: str, keep_genres, new_genres):
             else:
                 # For "Various Artists", pick the most popular track genre.
                 item_genres = []
+                assert isinstance(obj, Album)  # Type narrowing for mypy
                 for item in obj.items():
                     item_genre = None
                     if "track" in self.sources: