formula_cellar_checks: check Python site packages

[Bo98]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

This audit compares this list to the list of non-recursive dependencies.

Intended behaviours:

• Formula depends on python@3.8 and package is installed for Python 3.8 -> pass
  • If there are additional packages for other versions they are ignored. This allows formulae to supply packages for multiple Python versions.
• Python package for 3.7 installed but depends on python@3.8 -> fail
• Python package installed but no Python dependencies -> pass
  • Python packages may be an "optional" part of the formula.

When testing, dependencies are not searched recursively. It's difficult to guarantee zero false positives here, particularly if the dependency tree has mixed versions of Python.

[MikeMcQuaid]

Few suggested style changes but looks good, thanks again!

[MikeMcQuaid]

Thanks again @Bo98!