doc: add python2 intersphinx

Now we don't only search latest (= Python 3.4) docs for objects,
but also Python 2.7 docs. Some things are gone in Python 3.

Last item takes precedence, so Python 2 docs are only used
when some object isn't in Python 3 docs.

Not really sure if we should push users to Python 3,
but only using Python 3 docs is the example given on readthedocs.

[ci skip]