Updating image entropy implementations

[fish2000]

As of version 6.1.0, the Pillow imaging library’s C module includes an optimized native entropy method:

• https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst#610-2019-07-01

This change detects and selects this new native method, if it is available in the version of Pillow upon which easy-thumbnails finds itself running.

Additionally, an optimized version of the existing Python image-entropy method is furnished as a fallback: rather than summing the entire histogram value set, this new function calculates the product of the image dimensions and band count, arriving at the same number. The new function also uses generator expressions and the specialized math.fsum(…) and math.log2(…) library calls to improve on the performance of its predecessor without altering the algorithm.

The appropriate image-entropy function is then conditionally ensconced in the easy_thumbnails.utils module, at module-load time.

I also made a slight tweak to the pil_image function, found in easy_thumbnails.source_generators – the call to PIL.Image.Image.load() was raising the exception noted in comments in that functions’ source while I was running the testsuite; now all such calls have these exceptions swallowed and everything works OK.