Resolve import errors in Python3

The built-in interpreter in Sublime Text 3 raises 'ImportError'
exceptions if imports aren't relative to the 'Packages' directory.
The same issue doesn't occur in the standard Python3 interpreter.
Resolve the issue by hacking the path when running in Sublime Text.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>