formula_cellar_checks: skip unless Python libs

[jonchang]

This avoids an unusual error message when Python is needed for building (e.g., code generation) and files are installed to lib but Python is not used at runtime.

Needed for https://github.com/Homebrew/linuxbrew-core/pull/19996

==> brew audit recode --online
Error: 1 problem in 1 formula detected
recode:
  * Packages have been installed for:

    but this formula depends on:
      Python 3.7

Because build-time dependencies are uninstalled on test-bot, this audit shouldn't be triggered as no Python libraries are actually installed to lib.

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[Bo98]

Yeah that makes sense - my bad. Thanks!

[jonchang]

Thanks @Bo98!