[PATCH] py3: use setattr() to assign new class attribute

[Yuya Nishihara]

On Tue, 24 May 2016 19:24:23 +0530, Pulkit Goyal wrote:
>  Well I think I don't get you, I send this patch in relation to py3
> porting, and this thing will raise an error in py3 if not fixed.
> > FYI, thanks to the refactoring of tests-patchbomb.t, 7f1d263a1bcb, tests
> > passed on Python 2.6.9 without this hack.

Augie pointed out that we wouldn't need to wrap email.header.Header on
Python3, that's probably true. Related to this, I've tried quick hack to
see if we can eliminate the wrapper before, and I noticed it isn't covered by
tests. So if you're going to take a step further, be careful.