Fix flake8 code style errors in halos module

[rrjbca]

Description

Fix flake8 code style errors in halos module. Requires #430 to be merged first.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request