https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=955070 indicates that the python-doc8 package FTBFS.
I found that the python-oslosphinx is cause of the FTBFS because it is not compatible with Sphinx 2.4. python-oslosphinx itself doesn't FTBFS because it has insufficient testing.
I found that the python-doc8 upstream was deprecated https://git.openstack.org/cgit/openstack/doc8, but this page did appear to point to a continuation of the project at https://github.com/PyCQA/doc8.
At the new location, the project has removed usage of python-oslosphinx, and has an rc3 for a new 0.8.1 release.
By changing to this 0.8.1-rc3 release, the package would build again.
I don't know of a package that depends on python-doc8, so I wasn't able to verify the new version.