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.

[Christian Boos]

• Use ​ context managers and the `with` statement

The change concerning the context managers and the with statement mostly concerns the refactor transaction handling topic (#8751), as discussed in the ​googlegroups:trac-dev:4efe0b99b4e78d23 mail and follow-ups.

But we could also use it for other things, like holding locks.

Slightly related, a few places could also benefit from the unification of ​ try/except/finally.