Make python style compliant

Brings the python files in this repo into compliance with the
checks in `sel4_tools`.

Convert to SPDX license tags

This includes marking all documentation files CC-BY-SA-4.0.

python-capdl-tool: Refactor tests

- make the tests real python unit tests
- convert the bash runner to a python unit test runner
- place all tests under tests/ rather than nested dirs
- rename some artifacts