Add .isort.cfg to fix false positives on Travis

When run on Travis, `isort` thinks that `binascii` is a 3rd-party
module (it's part of the python standard library), and that `cv2` is a
standard library (it's a third-party library, from `apt-get install
python-opencv`). I don't know why.

I've raised https://github.com/timothycrosley/isort/issues/157