Compatibility fixes for Python 3.3

1) Added a py33 environment to tox
2) Updated tests to mock the correctly named builtins.open based on
python version
3) Other minor compatibility fixes

Tests for Python 3.3 will not pass due to this bug:
https://github.com/eventlet/eventlet/issues/83 among possibly others in
eventlet.

Change-Id: Ie4b512a926fa690ee77a71a89851c871ea1f6be0