[$] Leaving python-dev behind

It was not all that long ago that Python began its experiment with
replacing one of its mailing lists with a forum on its Discourse discussion site. Over
time, the Discourse instance has become more and more popular within the
Python community. It would seem that another mailing list will soon be
subsumed within Discourse as the Python steering council is planning to
effectively retire the venerable python-dev mailing list soon.