Mypy stub files for Cython files

It would be good to add [stub files](https://mypy.readthedocs.io/en/stable/stubs.html#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.