Exception occurred:
File "/builddir/build/BUILD/greenwave-0.9.4/docs/conf.py", line 211, in get
app.warn(msg.format(url, e))
AttributeError: 'Sphinx' object has no attribute 'warn'
This also fails with Python 3.7 (but still blocks the Python 3.8 rebiuild).
It's a Sphinx update problem, this has been deprecated since Sphinx 1.6 and removed in Sphinx 2.0, you should update to the logging API:
https://www.sphinx-doc.org/en/2.0/extdev/logging.html