Fix and simplify check-python dev script

[jotaen4tinypilot]

I revisited and simplified a few things in the check-python dev script, partially as follow-up of https://github.com/tiny-pilot/tinypilot-pro/pull/1714:

• Fix ADDITIONAL_PY_SCRIPTS (which was outdated) for a dynamic lookup mechanism, which ensures to always catch all scripts in scripts/.
• Consolidate check-for-init-python-files into the check-python script. It seemed weird to me to have this script separately and everything else embedded.
• Fix some bash quotes ("→')

This structure also makes it a bit easier to include the front panel sources here: I realized that the check-python script also doesn’t sufficiently account for the front panel. So for the upstream merge, I plan to add these changes (see this Pro branch I used for testing):

• 👉 In Pro’s CircleCI config, we have to install the front panel venv before running check-python. That actually triggers some missing ignore-directives (e.g., in .yapfignore), that also would show up when running locally.
• 👉 ruff doesn’t cover the front panel, so we have to add it there.
• ✅ yapf already covers the front panel.
• ❌ coverage doesn’t include the front panel – we don’t have any tests in the front panel, but I also wasn’t able to figure out how to make it even look there…
• ❌ pylint doesn’t include the front panel – unfortunately, I wasn’t able to make that happen trivially, so we should look into this separately – there are some minor style violations there.

I think the aforementioned problems would profit from a revised project structure, where we properly share dev dependencies for both our Python services. The current nested structure doesn’t seem to lend itself as nicely for this.