bpo-13601: Mention stderr's line buffering in What's New

[hauntsaninja]

https://bugs.python.org/issue13601

Automerge-Triggered-By: @gvanrossum

[hauntsaninja]

Alternative, terser phrasing from sys.rst:

Non-interactive stderr is now line-buffered instead of fully buffered.

I gave credit based on https://github.com/python/cpython/pull/17646/files#diff-c32d9a73be8db9e122317d9c23b6c0da — apologies if anyone else should be credited.

[hauntsaninja]

cc @gvanrossum

[gvanrossum]

There's one subtle point. This really only applies to the initial value of sys.stderr. If you assign something else to sys.stderr its buffering isn't magically going to change. But I'm not sure that most readers of this document would appreciate the extra words we'd need to add to make this distinction (since few people ever assign anything to sys.stderr), and to more advanced readers this would be obvious.

[gvanrossum]

Thinking about it some more, the place to note that subtle point would've been the Misc/NEWS entry for the original PR, and since that didn't, let's say we won't need it here. So I'll mark this as automerge and go to bed.

[hauntsaninja]

The "defaults to" phrasing helps hedge that (and -u), too, since if you assign to sys.stderr you aren't using the default.