The problem is in Pillow, 3.4.0 through 3.4.2. I have a workaround for the OpenSlide Python test suite that I'll get committed and then post here.
- Bug#852502: openslide-python: FTBFS (failing tests) Santiago Vila
- Bug#852502: openslide-python: FTBFS (failing tests) Santiago Vila
- Bug#852502: openslide-python: FTBFS (failing tests) Benjamin Gilbert
- Bug#852502: openslide-python: FTBFS (failing tes... Benjamin Gilbert