python-magic-wormhole-mailbox-server fails to build with Python 3.9.0a4.

+ /usr/bin/python3 setup.py build '--executable=/usr/bin/python3 -s'
Traceback (most recent call last):
  File "/builddir/build/BUILD/magic-wormhole-mailbox-server-0.3.1/setup.py", line 20, in <module>
    long_description=open('README.md', 'rU').read(),
ValueError: invalid mode: 'rU'

See https://docs.python.org/3.9/whatsnew/3.9.html#changes-in-the-python-api

"open(), io.open(), codecs.open() and fileinput.FileInput no longer accept 'U' (“universal newline”) in the file mode. This flag was deprecated since Python 3.3. In Python 3, the “universal newline” is used by default when a file is open in text mode. The newline parameter of open() controls how universal newlines works."


For the build logs, see:
https://copr-be.cloud.fedoraproject.org/results/@python/python3.9/fedora-rawhide-x86_64/01260340-python-magic-wormhole-mailbox-server/

For all our attempts to build python-magic-wormhole-mailbox-server with Python 3.9, see:
https://copr.fedorainfracloud.org/coprs/g/python/python3.9/package/python-magic-wormhole-mailbox-server/

Testing and mass rebuild of packages is happening in copr. You can follow these instructions to test locally in mock if your package builds with Python 3.9:
https://copr.fedorainfracloud.org/coprs/g/python/python3.9/

Let us know here if you have any questions.

Python 3.9 will be included in Fedora 33. To make that update smoother, we're building Fedora packages with early pre-releases of Python 3.9.
A build failure prevents us from testing all dependent packages (transitive [Build]Requires), so if this package is required a lot, it's important for us to get it fixed soon.
We'd appreciate help from the people who know this package best, but if you don't want to work on this now, let us know so we can try to work around it on our side.