When a rm operation is performed, including indirectly through a mv, the optimized batched version of the command is executed (property on_google=True).
However, possibly due to updates on google sides, in case of requester_pays=True mode Google complains that the userProject is not provided for billing. I debugged and checked it was there, but the batch version does weird stuff with the header so I don't really know what's going on.
Anyway I get this error:

OSError: {    "code": 400,    "message": "Bucket is a requester pays bucket but no user project provided.",    "errors": [      {        "message": "Bucket is a requester pays bucket but no user project provided.",        "domain": "global",        "reason": "required"      }    ]  }

If I change the code to avoid the _rm_files method (setting the property on_google=False by hand) it works.

Seems like a bug, due to changes on google sides. Here to let you know.
Thanks for your work!