Idea: mention 3rd party "extended stdlib" modules in developer guide

We have a section in the developer guide specifically on proposing changes to the standard library: http://cpython-devguide.readthedocs.io/en/latest/stdlibchanges.html

One of the alternatives to standard library additions that frequently needs to be brought up on python-ideas is the possibility of adding to one of the "stdlib enhancement" projects that adds assorted general purpose programming utilities that are nevertheless considered sufficiently niche use cases that they don't make the cut for standard library inclusion (or, alternatively, folks want them now, rather than having to wait for a new CPython feature release)

Perhaps it would make sense to specifically mention some of those projects in this part of the developer guide? The main two that come to mind are:

• boltons: https://boltons.readthedocs.io/
• more-itertools: https://pypi.python.org/pypi/more-itertools/

There are probably others, but once we start the list, it's relatively easy to add new entries, but with no list at all, it's unlike anyone (else) would suggest adding one.

Beginne mit dem Abschnitt zu Änderungen an der Standardbibliothek im Developer Guide unter stdlibchanges.html und prüfe, wie alternative Projekte derzeit beschrieben werden. Füge Hinweise auf boltons und more-itertools hinzu und bewahre dabei den bestehenden Stil des Abschnitts; abgeschlossen ist die Aufgabe, wenn der Guide die Leser auf diese Projekte als Alternativen zu Ergänzungen der Standardbibliothek verweist.