[mod] enable dict_comprehension tests on python2.6

Since now we don't rely on ast.py anymore, parsing those dict
comprehesions with a python2.6 interpreter is not a problem anymore
[mod] avoid unecessary indent
[mod] make future tests more understandable
[mod] conform to imports at the top of the module
[fix] take all future import grammar into account

See https://docs.python.org/2/reference/simple_stmts.html#future