AttributeError: 'tuple' object has no attribute 'format'

[rth]

I get the following error when trying to use this package on readthedocs Sphinx 3.5.4,

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pyodide/envs/latest/lib/python3.8/site-packages/sphinx/events.py", line 111, in emit
    results.append(listener.handler(self.app, *args))
  File "/home/docs/checkouts/readthedocs.org/user_builds/pyodide/envs/latest/lib/python3.8/site-packages/versionwarning/signals.py", line 40, in generate_versionwarning_data_json
    message=message.format(
AttributeError: 'tuple' object has no attribute 'format'

Full build log: https://readthedocs.org/projects/pyodide/builds/13659330/

Maybe it's a sphinx version compatibility issue?