Support for 4-bit greyscale TIFF images

 * Test image from depth/flower-minisblack-04.tif of ftp://ftp.remotesensing.org/pub/libtiff/pics-3.8.0.tar.gz. The directory's README.txt says:

> These sample TIFF image files are prepared by Bob Friesenhahn ... using a development version of GraphicsMagick 1.2. ...

> These files are hereby placed in the public domain.

 * Add `(II, 1, 1, 1, (4,), ()): ("L", "L;4")` to TiffImagePlugin.py's `OPEN_INFO` dict to support it.
 * With test case.
 * flake8.
 * Closes #932.