xtl fails to build with Sphinx 4.0.2 in Fedora 35 (currently Rawhide).

sphinx-build-3 -b html -d build/doctrees   source build/html
Running Sphinx v4.0.2
making output directory... done
Exception occurred:
  File "/builddir/build/BUILD/xtl-0.7.2/docs/source/conf.py", line 19, in setup
    app.add_stylesheet("main_stylesheet.css")
AttributeError: 'Sphinx' object has no attribute 'add_stylesheet'

For the logs from testing build attempts, see:
https://copr.fedorainfracloud.org/coprs/ksurma/pygments-2.9.0/package/xtl/

Possible solution to this issue:
In some cases it may be enough to replace the call to 'add_stylesheet' with 'add_css_file' instead.

You can test you package using Koji scratch build
koji build --scratch <srpm>

For details regarding the Sphinx change, see https://fedoraproject.org/wiki/Changes/Sphinx4
Let us know here if you have any questions. Thank you!