One review comment: the patch adds a new exception class that is used for the errors that are now additionally detected.  Elsewhere plistlib uses non-specific exception classes like ValueError.  If starting from scratch, it might be better to consistently use a specific exception class but that would create incompatibilities if changed now.  I don't see a compelling need to add one now just for these errors.  (But, if kept, it should be added to the docs.)  Otherwise, looks good to me.

Thanks for taking this on!