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.
Task description
- Add a teststuite image with Python 3.7 to buildstream-docker-images - buildstream-docker-images#29 (closed)
- Add that image to our CI pipeline
Acceptance Criteria
BuildStream's CI covers Python 3.7