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.

从开发者指南中 stdlibchanges.html 的标准库变更部分开始，检查目前如何描述替代项目。添加对 boltons 和 more-itertools 的提及，同时保留该部分现有的风格；当指南将读者指向这些项目，并将它们作为标准库新增内容的替代方案时，即表示完成。