doc: use a list for html_sidebars

Sphinx 1.7 deprecated the use of a single string in html_sidebars, and
Sphinx 2.0 has dropped support for this altogether.

Author: ktdreyer

doc/conf.py

@@ -98,10 +98,8 @@
 
 # html_index = 'index.html'
 html_sidebars = {
-    'index': 'indexsidebar.html',
+    'index': ['indexsidebar.html'],
 }
-#                 'basics': 'indexsidebar.html',
-# }
 # html_additional_pages = {'index': 'index.html'}
 
 # Theme options are theme-specific and customize the look and feel of a theme