Update code coverage + formating

[JonathanWoollett-Light]

Summary of the PR

• Applies standard black formatting to Python code.
• Updates code coverage error message to be more descriptive.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[JonathanWoollett-Light]

@aghecenco Could you please re-review.