Add stubs for importlib.__init__

[brettcannon]

Part of #189

[gvanrossum]

Instead, you could run

git config --global --add core.excludesfile ~/.gitignore_global

and create a ~/.gitignore_global file that does this for all repos you use!

[gvanrossum]

Merging this as-is, if you feel compelled to change things you can do that in the next step. :-)

[gvanrossum]

Hm... Tricky. Maybe you can just leave the method out of the ABC altogether? Or I would just allow certain calls that should be ruled out and live with it. Otherwise, experimentally it seems that mypy is happy with the subclass that leaves path out altogether. But I don't understand the asymmetry, and I wouldn't count on it. I'm guessing python/mypy#1237 is also relevant here.

[brettcannon]

I'll just leave the method out since the entire ABC has been deprecated since Python 3.4.