DOC: RTD build has some RemovedInSphinx90Warning warnings

[pllim]

I think this is coming from readthedocs_ext so maybe we just wait it out?

readthedocs_ext/readthedocs.py:106: RemovedInSphinx90Warning: 'sphinx.builders.html.StandaloneHTMLBuilder.css_files' is deprecated. Check CHANGES for Sphinx API modifications.
  if inject_css and theme_css not in app.builder.css_files:
readthedocs_ext/readthedocs.py:106: RemovedInSphinx90Warning: The str interface for _CascadingStyleSheet objects is deprecated. Use css.filename instead.
  if inject_css and theme_css not in app.builder.css_files:
readthedocs_ext/readthedocs.py:120: RemovedInSphinx90Warning: 'sphinx.builders.html.StandaloneHTMLBuilder.script_files' is deprecated. Check CHANGES for Sphinx API modifications.
  hasattr(app.builder, 'script_files'),
readthedocs_ext/readthedocs.py:121: RemovedInSphinx90Warning: 'sphinx.builders.html.StandaloneHTMLBuilder.script_files' is deprecated. Check CHANGES for Sphinx API modifications.
  js_file not in app.builder.script_files,
readthedocs_ext/readthedocs.py:121: RemovedInSphinx90Warning: The str interface for _JavaScript objects is deprecated. Use js.filename instead.
  js_file not in app.builder.script_files,

xref readthedocs/readthedocs-sphinx-ext#134