Mypy stub files for Cython files

[arcondello]

It would be good to add stub files for some of the most commonly used cython packages because cython current does not support type annotations.

Especially dimod.cyvariables and dimod.cyutilities would benefit because the Variables object is used in many places.

Should be as simple as adding a dimod/cyvariables.pyi file etc.

[arcondello]

As a downside, it does not seem like those type annotations are picked up by sphinx (see https://github.com/sphinx-doc/sphinx/issues/7630).