exthelper: drop the addattr() decorator Yuya pointed out that this goes against the typical advice to not add attributes to classes[1]. The evolve extension still uses this a handful of times, so maybe it should be brought back in the future if a general use is found. But it isn't nice to have a new helper API that can lead to easy problems. [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-December/126330.html