Make 'typing' package Python 2 compatible

[vlasovskikh]

The current typing module doesn't support Python 2 as well as the third-party backports.typing module. It's a good idea to have the source code of the typing module installed on your system when you type check Python 2 code, e.g. against Python stub files with types. It will simplify things for a static type checker.

I guess we could create a backported version of typing. I mean something similar to 3.5.0b1 with all the recent API changes. The 2/3 single source approach won't work here because the module heavily relies on metaclasses and uses type hints in many places.

I will start working on a backport in a separate branch.

[JukkaL]

Mypy has a somewhat functional Python 2 port of typing as well (https://github.com/JukkaL/mypy/blob/master/lib-typing/2.7/typing.py), though others could be more feature-complete (haven't checked).