ENH: using python's warnings instead of prints in fitpack.

When fitpack detects a recoverable problem, it will signal it with
a RuntimeWarning instead of printing to stdout. This way the output
can be programatically silenced or made exceptions.

There is one failure on Travis, but I don't see what has failed.
It is boilerplate at the top of every python file for python3 compatibility.  I think there's even a bot that enforces it, or that added them at some point.