doc build fails with sphinx 4: no attribute 'add_stylesheet'

[drew-parsons]

The doc build for xtensor-blas fails with sphinx 4:

sphinx-build -b html -d build/doctrees   source build/html
Running Sphinx v4.2.0
making output directory... done

Exception occurred:
  File "/build/xtensor-blas-0.19.1/docs/source/conf.py", line 19, in setup
    app.add_stylesheet("main_stylesheet.css")
AttributeError: 'Sphinx' object has no attribute 'add_stylesheet'
The full traceback has been saved in /tmp/sphinx-err-dacahox3.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
make[2]: *** [Makefile:56: html] Error 2

This is the same as xtensor Issue #2441, xtensor-stack/xtensor#2441

add_stylesheet is deprecated and needs to be replaced with add_css_file()