Update Dockerfiles and CI scripts for Python3 tests

User Story
AS a Developer,
WHEN I run CI
THEN it successfully attempts to run the tests using Python 3.6
AND I can build test images which support Python 3.6

Notes

• The scope of this work does not include ensuring the tests pass
• At the time of writing, the tests fail to run because it is blocked at the "build" step by the explicit use of Python 2.7 in the CI scripts and test-related Dockerfiles