PKG: remove pyproject.toml for now
Since this gives problems, it might be better to remove this until support in pip is better? 
Eg I also have errors on the geopandas CI when installing pandas master with pip due to this.

Scipy did something similar: https://github.com/scipy/scipy/pull/8735
is this what is causing the wheel build issues? 

 That's still something else I think. It's also related to the release of pip 10, but because they changed some api's that multibuild relied upon: https://github.com/matthew-brett/multibuild/issues/159 cc @TomAugspurger  Going to merge this, as currently it is just broken. We can always add back before the 0.23.0 release if pip is updated by then.