ci: Re-enable flake8's F401 rule: no unused imports

[romanc]

Description

Follow-up PR from #159, which accidentally added an exception for F401 (no unused imports) for all files. It turns out, we can re-enable this rule for all files if we populate the __all__ variable in __init__.py files, simplifying configuration.

I've also commented on why we have the other exceptions and grouped them by "redundant with black" and "clashing with NDSL/stencil syntax".

How Has This Been Tested?

Running pre-commit locally.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included

[FlorianDeconinck]

Nice.