Replace display_version with version_selector

In sphinx-rtd-theme version 3.0.0, `display_version` was removed in favor
of `version_selector` and `language_selector`.

src/conf.py

@@ -66,7 +66,7 @@
 html_static_path = ['static']
 
 html_theme_options = {
-    'display_version': False,
+    'version_selector': False,
     'subproject': False, # For new theme versions
     'logo_only': True,   # For old theme versions
     'collapse_navigation': False,