Having this call there was necessary in the past as the open() method would
need the canvas size as a parameter. This is now determined automatically
after rendering.
Still calling open() is deprecated but won't result in an error.
- Use /usr/bin/env to determine path to python3 instead of a hard-coded
path, so that virtualenv stubs work properly.
- Change package name to 'boxes' ('boxes.py' did not work within
virtualenv).
Remove width and height parameter from Boxes.open()
Code for detecting the size ignores text. Until this is fixed you need
to surround text by lines of some sort.