Support for 4-bit greyscale TIFF images

 * Test Hopper image created with ImageMagick: `convert tests\images\hopper.jpg -colorspace Gray -colors 16 -depth 4 tests\images\hopper_gray_4bpp.tif`
 * Add `(II, 1, 1, 1, (4,), ()): ("L", "L;4")` to TiffImagePlugin.py's `OPEN_INFO` dict to support it.
 * With test case that compares image is similar to an original.
 * flake8.
 * Replaces #933.
 * Closes #932.