Python 3: Fix import path for d6message in deepsix

This patch fixes the import path to be the full path for d6message.
This was causing an ImportError for python3. In addition, we should
have been using the full path anyways. Lastly, this import should not
be grouped with the stdlibs.

Closes-bug: 1403209
Partially implements blueprint: support-python3

Change-Id: I0d721446401225523b7acbab0a3514b53056f815
Merge "Python 3: Fix import path for d6message in deepsix"