deprecation warnings on theme

[aaronmarkham]

Sphinx 2.0 will break some things... since the Windows setup uses 2.0 maybe things are already breaking when running there...

/home/ubuntu/anaconda3/envs/mxnet-cuda10-docs/lib/python3.7/site-packages/sphinx/builders/html.py:1171: RemovedInSphinx20Warning: Now html_sidebars only allows list of sidebar templates as a value. Support for a string value will be removed at Sphinx-2.0.
  self.add_sidebars(pagename, ctx)
/home/ubuntu/anaconda3/envs/mxnet-cuda10-docs/lib/python3.7/site-packages/sphinx/builders/html.py:138: RemovedInSphinx20Warning: builder.css_files is deprecated. Please use app.add_stylesheet() instead.
  ret += other