pil_cv2.check_dpi: fix class membership test

[bertsky]

(depending on how the PIL.Image was instantiated – file plugin or array interface – the previous isinstance could fail, provoking a fall-through to cv2pil which does not work)

example: PIL.JpegImagePlugin.JpegImageFile instances do not match, causing error message from cv2pil → cvtColor:

OpenCV(4.6.0) :-1: error: (-5:Bad argument) in function 'cvtColor'
> Overload resolution failed:
>  - src is not a numpy array, neither a scalar
>  - Expected Ptr<cv::UMat> for argument 'src'