Remove Python 2.4 compatibility

[Remy Blank]

The goal of this task is to remove Python 2.4 compatibility code, and to start using Python 2.5-only features. This includes:

• Use context managers and the with statement.
• Use ... if ... else ... expressions instead of the more error-prone ... and ... or ....
• Remove our own implementations for functionality that was introduced in Python 2.5.
• Use generator expressions instead of list comprehensions where adequate.

Please add new items to this list as they pop up.