setup: include vendored 3rd party type stubs While pytype may not support PEP 561, PyCharm does, so having the stubs available means it can determine `foo = attr.ib(type=int)` means `foo` is an int. This only applies when using Mercurial as a library, like with TortoiseHg development- PyCharm is already smart enough to use the *.pyi files in the Mercurial source tree when hacking on Mercurial itself. I left the mercurial.cext stubs out because it seems very low level, that 3rd parties shouldn't be using directly.