enable support for more recent versions of sphinx for documentation generation

Looks like plotly documentatio ncannot be generated using latest sphinx 61.3.

+ /usr/bin/sphinx-build -n -T -b man doc/apidoc build/sphinx/man
Running Sphinx v6.1.3
WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).
making output directory... done

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/cmd/build.py", line 279, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/usr/lib/python3.8/site-packages/sphinx/application.py", line 237, in __init__
    self.config.setup(self)
  File "/home/tkloczko/rpmbuild/BUILD/plotly.py-5.13.1/doc/apidoc/conf.py", line 209, in setup
    app.add_stylesheet("plotly-style.css")  # also can be a full URL
AttributeError: 'Sphinx' object has no attribute 'add_stylesheet'

Exception occurred:
  File "/home/tkloczko/rpmbuild/BUILD/plotly.py-5.13.1/doc/apidoc/conf.py", line 209, in setup
    app.add_stylesheet("plotly-style.css")  # also can be a full URL
AttributeError: 'Sphinx' object has no attribute 'add_stylesheet'
The full traceback has been saved in /tmp/sphinx-err-cu42fgnj.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!

The failure is in doc/apidoc/conf.py at setup, where Sphinx 6.1.3 lacks the referenced API. Start by reading that setup and run the shown sphinx-build command; done means documentation generation completes with Sphinx 6.1.3 without the AttributeError.