graphics/py-mcomix: Fix runtime with graphics/py-pillow 6.0

• Prevent runtime breakage when graphics/py-pillow is updated to 6.0 where the deprecated constant "Image.VERSION" is no longer available. [1]

• Add a workaround for the behavior of the "_getexif" function that is still buggy with PNG files that contain EXIF meta data. [2]

Also while I'm here:

• Add "gnome" to USES as using USE_GNOME alone is deprecated.

PR: 237887, 237426 (related)
Approved by: ashish (maintainer)
Obtained from: upstream [1], https://github.com/multiSnow/mcomix3/issues/76 [2]