Add special treatment for ZYTE_API_RETRY_POLICY

[Gallaecio]

Unlike most settings accepting an import path string, this setting does not support that import path string being replaced with the actual object, because the actual object is a tenacity object that is not picklable, and settings must be picklable.

So, we need to make sure we do not flag import path strings as an issue for this specific setting, and instead make sure we do the opposite and flag objects (unless flagging objects seems too prone to false positives).