Yep, that's the one. You can define a some helper functions at the top level like this:
- Python 3: https://bitbucket.org/gutworth/six/src/e3da7fd96039a6ed89493f89d121c4f3797e6713/six.py?at=default#cl-345
- Python 2: https://bitbucket.org/gutworth/six/src/e3da7fd96039a6ed89493f89d121c4f3797e6713/six.py?at=default#cl-368

Then call this raise function inside _reraise.

Don't bother to setup a Python 3 env if you don't already have one, just make the changes following your intuition and see if Travis if fine with them ;-)