libmypaint fails to build with Sphinx 3.0.0.

WARNING: html_static_path entry '_static' does not exist
WARNING: failed to reach any of the inventories with the following issues:
intersphinx inventory 'http://docs.python.org/objects.inv' not fetchable due to <class 'requests.exceptions.ConnectionError'>: HTTPConnectionPool(host='docs.python.org', port=80): Max retries exceeded with url: /objects.inv (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fea57800520>: Failed to establish a new connection: [Errno -2] Name or service not known'))
/builddir/build/BUILD/libmypaint-1.5.0/doc/source/index.rst:17: WARNING: Invalid C++ declaration: Expected end of definition. [error at 21]
  typedefG_BEGIN_DECLS struct MyPaintGeglTiledSurface MyPaintGeglTiledSurface
  ---------------------^

Exception occurred:
  File "/usr/lib/python3.8/site-packages/sphinx/domains/c.py", line 3053, in object_type
    raise NotImplementedError()
NotImplementedError

For the build logs, see:
https://copr-be.cloud.fedoraproject.org/results/cstratak/sphinx_3.0.0/fedora-rawhide-x86_64/01335486-libmypaint/

For all our attempts to build libmypaint with Sphinx 3.0.0, see:
https://copr.fedorainfracloud.org/coprs/cstratak/sphinx_3.0.0/package/libmypaint/

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 Sphinx 3:
https://copr.fedorainfracloud.org/coprs/cstratak/sphinx_3.0.0/

Let us know here if you have any questions.

Sphinx 3 will be included in Fedora 33. To make that update smoother, we're building the dependent packages in Copr.
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.