Created with ImageMagick: convert tests\images\hopper.jpg -colorspace Gray -colors 16 -depth 4 tests\images\hopper_gray_4bpp.tif
Support and test for 4-bit greyscale TIFF
flake8
Added 4bit test to libtiff as well
Merge pull request #980 from wiredfool/pr_938

4 bit TIFF support