Drop the monolithic doc string
Drop the monolithic doc string

We now have full sphinx docs as per #14.
pluggy.py is the entire project and it doesn't make much sense to
describe that module's contents other then saying "this is it folks".

RonnyPfannschmidt/pluggy,RonnyPfannschmidt/pluggy,pytest-dev/pluggy,nicoddemus/pluggy,hpk42/pluggy,pytest-dev/pluggy,tgoodlet/pluggy