Use Pillow instead of PIL (rebased onto develop)



This is the same as gh-2077 but rebased onto develop.

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PIL is unofficially not maintained anymore and its installation via pip 1.5 or
later now requires to pass --allow-external PIL --allow-unverified PIL
The proposal of this commit is to use Pillow as a replacement.

/cc @manics @knabar @will-moore @aleksandra-tarkowska