Use ReadTheDocs theme in sphinx docs.

Looks like readthedocs does not like 'classic' theme.

docs/conf.py

@@ -110,7 +110,7 @@ pygments_style = 'sphinx'
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'classic'
+html_theme = 'sphinx_rtd_theme'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the