Include import of __future__ features division, absolute_imports and
print_function everywhere. Don't add unicode_literals yet for it is
harder to convert.
Goal is smoothing the transition to python 3.
Don't mutate the base classes. This was causing plugins to break on the second
test that uses them, since after they were unloaded their class-level fields
were broken (since the modules are not re-loaded).
This makes it more clear than ever that we need to encapsulate plugin loading
state using some manner of PluginManager.
