docs/conf: Reference CPython 3.5 docs.

CPython 3.6 contains some backward incompatible changes, and further
version(s) are expected to have more. As we anyway implemente 3.4 with
some features of 3.5, refer to 3.5 docs to avoid confusion.

Examples of 3.6 backward incompatibilities:

https://docs.python.org/3.6/library/json.html#json.dump
https://docs.python.org/3.6/library/json.html#json.load

> Changed in version 3.6: All optional parameters are now keyword-only.

https://docs.python.org/3.6/library/functions.html#type

> Changed in version 3.6: Subclasses of type which don’t override
> type.__new__ may no longer use the one-argument form to get the
> type of an object.

https://docs.python.org/3.6/library/collections.html#collections.namedtuple

> Changed in version 3.6: The verbose and rename parameters became
> keyword-only arguments.

docs/conf.py

@@ -322,7 +322,7 @@ texinfo_documents = [
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'python': ('http://docs.python.org/3', None)}
+intersphinx_mapping = {'python': ('http://docs.python.org/3.5', None)}
 
 # Append the other ports' specific folders/files to the exclude pattern
 exclude_patterns.extend([port + '*' for port in ports if port != micropy_port])