Add tests for python 3.7

Background

We aim to support python3.7 but none of our testsuite images have it covered at the moment.

If we had tests for python3.7, there may be some hope of capturing things like #836.

Add a teststuite image with Python 3.7 to buildstream-docker-images - buildstream-docker-images#29 (closed)
Add that image to our CI pipeline

BuildStream's CI covers Python 3.7

Edited Jan 15, 2019 by Chandan Singh

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information