BUG : fix list comprehensions over class members

Due to scoping fixes in py3k, list comprehensions over class level
attributes during class definition does not work (see
http://stackoverflow.com/questions/13905741/accessing-class-variables-from-a-list-comprehension-in-the-class-definition).

Superficially Fixes #3436. There seem to be other issues
How can this work? `_font_awesome_classes` now appears only here in this file, so as far as I can see, it is undefined. Overall, this block of code looks like a list incomprehension, not a list comprehension.