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
Allow multiple extensions to be loaded by switching to ExtensionManager
from stevedore. Remove any reference to modes.
Change-Id: Ic160478625226b4dd17bd68b3d37f3b05823e519