Use generic Python 3 and 2 URLs in intersphinx

This change allows the docs link against the latest Python 3
references as well as against some outdated Python 2 refs where
necessary.

Before this commit, the docs were only linkable against Python 3.6.

Author: webknjaz

source/conf.py

@@ -352,7 +352,8 @@
 
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
-    'python': ('https://docs.python.org/3.6', None),
+    'python': ('https://docs.python.org/3', None),
+    'python2': ('https://docs.python.org/2', None),
     'pip': ('https://pip.pypa.io/en/latest/', None),
 }