Add tests for python 3.7
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