docs/conf.py: Use https for intersphinx link to docs.python.org.

To get rid of warning when building the docs saying there's a redirect from
http: to https:.

docs/conf.py

@@ -298,4 +298,4 @@
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'python': ('http://docs.python.org/3.5', None)}
+intersphinx_mapping = {'python': ('https://docs.python.org/3.5', None)}