Depends on Pillow instead of PIL (closes #4451119)

The Pillow library is becoming the de-facto replacement for PIL.
It also is way simpler to install with pip than PIL.

Putting it as a dependency allows pip to be happier, without forcing older
distributions still shipping PIL to switch.

__pkginfo__.py

@@ -22,7 +22,7 @@ classifiers = [
 
 __depends__ = {'cubicweb': '>= 3.19.0',
                'pycrypto': None,
-               'PIL': None,
+               'Pillow': None,
                }