Using ImageSequence.Iterator on a non-stack SPIDER image leads to infinite loop.
EOFError (which stops the iteration) is never raised because when the image isn't a stack,
seek() returns gently without error.
In py3k, imports are absolute unless using the "from . import" syntax.
This commit also solves a recursive import between Image, ImageColor, and
ImagePalette by delay-importing ImagePalette in Image.
I'm not too keen on this commit because the syntax is ugly. I might go back
and prefer the prettier "from PIL import".
