.config/rss2email.cfg: Change python-pep to python-checkins

.config/rss2email.cfg: Change python-pep to python-checkins

I want to get the changes as well as the commit messages (which can
occasionaly be cryptic).  On peps@python.org, Barry Warsaw suggested:

On Sun, Oct 21, 2012 at 04:22:28PM -0400, Barry Warsaw wrote:
> On Oct 20, 2012, at 10:53 AM, W. Trevor King wrote:
> >I'm interested in following a few packaging PEPs, but not
> >interested enough to subscribe to python@ and python-dev@.
>
> Maybe as an alternative you can follow the checkins list by Gmane.
> You'll get everything committed to all the Python repos, but it
> shouldn't be too difficult to hunt down the PEP checkins.
>
> http://dir.gmane.org/gmane.comp.python.cvs

Finally change my rss2email configuration over to use his suggestion.