`Tileset.tiles` return annotation is incorrect

The `tiles` method is annotated as returning a list of dicts:

* https://github.com/higlass/higlass-python/blob/main/src/higlass/tilesets.py#L171
* https://github.com/higlass/higlass-python/blob/main/src/higlass/tilesets.py#L223

but the handler that consumes it actually unpacks (tile_id, tile) pairs:

* https://github.com/higlass/higlass-python/blob/main/src/higlass/_widget.py#L83

Returning `list[dict]` makes the dict comprehension raise an error that gets swallowed (see companion issue) so no response is sent and the UI reports "Tileset info not found".