python doc: "repl" and "setup-py" are goals now, don't use "py". fix 404 link

* rm 'pants py' mentions
* mention `goal repl`, `goal setup-py`
* how to pass args
* show `goal test` in covg section
* fix 404 link
* readability fixes

Testing Done:
rendered at http://pantsbuild.github.io/lahosken-demo/python-readme.html

Bugs closed: 688

Reviewed at https://rbcommons.com/s/twitter/r/1302/