Add basic Python 3 tests

This uses `nose' to load and run tests for the time being, as
testrepository tries to load everything, which is not possible
currently. Until everything is Python 3 compatible, using `nose' will be
simpler to list the specific files we want to run.

Blueprint: make-python3-compatible

Change-Id: I4c8dbf11ac8c6e0f25bc60a3e880a30bf87a60bd
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Add basic Python 3 tests"