[docs] New link for CodeTools

[Robert Kern]

The Python 3 docs
<http://python.readthedocs.org/en/latest/_sources/library/collections.txt>
link to a file in an old repository.

      * The `MultiContext class
        <http://svn.enthought.com/svn/enthought/CodeTools/trunk/enthought/contexts/multi_context.py>`_
        in the Enthought `CodeTools package
        <https://github.com/enthought/codetools>`_ has options to support
        writing to any mapping in the chain.

Should be this:

      * The `MultiContext class
        <https://github.com/enthought/codetools/blob/4.0.0/codetools/contexts/multi_context.py>`_
        in the Enthought `CodeTools package
        <https://github.com/enthought/codetools>`_ has options to support
        writing to any mapping in the chain.

That link should persist more reliably in the future.

Thank you!

-- 
Robert Kern
Enthought