Link to python 3 docs

Author: asottile

docs/conf.py

@@ -58,6 +58,8 @@
 # The full version, including alpha/beta/rc tags.
 release = version
 
+intersphinx_mapping = {'python': ('https://docs.python.org/3', None)}
+
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
 #language = None