Convert pyflakes to use the Python 2.5+ _ast module instead of the older (now essentially unmaintained) compiler package.
Introduce a number of new tests for various edge cases previously untested, as well, since this involved changing substantial chunks of pyflakes internals.
Also add support for certain new constructs which will be added in Python 2.7, including set comprehensions and dict comprehensions.
Because Python 2.4 does not include the _ast module, this change effectively drops support for running Pyflakes using Python 2.4.