Resolve some minor packaging issues

Confusingly, python has two options for packaging, the default distutils and the improvement setuptools. The setup.py script of pesummary uses several options from setuptools which aren ot available in distutils. Users may have noticed warnings such as Unknown distribution option: 'entry_points' while running setup.py install. I believe this was causing some confusion around the installed command line tools.

This MR switches to using setuptools and cleans a few minor things in the setup.py script. Testing on a clean environment, I see summarypages installed (and not summarypages.py).

For current users, this change may require them to "clean out" there pesummary installation. Instructions for how to do this can be found here: https://lscsoft.docs.ligo.org/bilby/installation.html#common-problems-multiple-bilby-versions (replacing bilby with pesummary of course).