Unify PiPy and setuptools installation

Don't repeat ourselves in the requirements.txt.

I wanted to check with @matze before merge, maybe he knows about some hidden pitfalls of this.
It will make our life _much_ easier when we will want to install dependencies specific for a certain python version, like the enum discussed [here](https://github.com/ufo-kit/concert/commit/22d433dea1dec9adac856744b21e20d90fc77570).