NEWS, python: update pointer to online documentation

Currently, http://packages.python.org/notmuch/ goes through a series
of redirections and ends up pointing to readthedocs.  Since we're
using readthedocs directly anyway, just point to it directly.

readthedocs are also now sensibly using a separate domain
(readthedocs.io) for their hosted documentation as distinct from their
own domain (readthedocs.org), so use the correct tld.