nipap: add a requirements.txt file for nipapd

pip install -r requirements.txt to install all (development) dependencies. So
far this only works for python 2.
As far as I understand the requirements.txt file it describes in a very precise way what packages an application you deploy depends on. If you deploy an applicaton you want to pin the dependency to a set of dependencies that really works. The problem is that most libs have quite liberal dependencies. If we only list the actuall dependencies maybe a lib sneaks in that breaks the app. Maybe this is a good resource on this topic: https://caremad.io/blog/setup-vs-requirement/

I created the file with `pip freeze` which is the standard why of creating the file. Of course we can thinker with the file afterwards but I don't think we should do this.