Mapping.{get,pop} can return default type

[gnprice]

See #223. That PR was reverted because it caused some type errors that it shouldn't, but it's the right thing to do and we should figure out how to make it work.

@gvanrossum , what did the errors you saw (after python/mypy#1595 was fixed) look like?

[rwbarton]

See also python/mypy#738 and python/mypy#1114.

[euresti]

If I understand correctly this looks fixed by #822
@gnprice: Is it what you wanted?

[JelleZijlstra]

I think it's fixed for get but not pop.

[euresti]

Oh yeah. And we use the pop version a lot. That sounds like something I can help out with.