PyYaml > 6 requires loader argument

[jack89roberts]

Just did this for the Turing fork of the material. Replace yaml.load/dump with safe_load / safe_dump. See yaml/pyyaml#576.

[jack89roberts]

Note we have a reworked version of this repo here for the Turing version of the course:
https://github.com/alan-turing-institute/bad-boids

The main difference is that the original version here requires the contents of fixture.yml to be changed for the tests on the better_boids branch to pass, which students found confusing as we talk about refactoring changing the style of the code but not the results. Also adds some niceties like a black and flake8 actions workflow.