Support for Python 2.6+ *and* Python 3 via ``six``.

Support for Python 2.5 was dropped since Sphinx only runs on
Python 2.6+ and supporting Python 2.5 requires some ugly
workarounds since it does understand neither of those::

    from __future__ import print_function

    try:
        raise Exception()
    except Exception as exc:
        pass
Update ``.gitignore``.