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. See ​what's new in Python 2.5. This includes:

• Use context managers and the with statement for: database transactions, handling locks, handling files.
• Use ... if ... else ... expressions instead of the more error-prone ... and ... or ....
• Use try: except: finally: instead of nested try: finally: and try: except:.
• Use except Exception: instead of except:.
• Remove our own implementations for functionality that was introduced in Python 2.5.
• Use generator expressions instead of list comprehensions where adequate.
• Use tuple arguments to str.startswith() and str.endswith().
• Use relative imports for parent, sibling and child modules?

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.

[Christian Boos]

• Use ... if ... else ... expressions instead of the more error-prone ... and ... or ....

Btw, Genshi supports this at least since the merge of the AST branch (see ​log:trunk/scripts/ast_generator.py) nearly 2 years ago, so we can also use that in templates (same thing for generator expressions).

[Remy Blank]

Copying items to the description, to get a nice overview. Keep 'em coming!

We should probably also start using except Exception: instead of except:.

What about using relative imports for parent, sibling and child modules (e.g. use from .api import ...)? I wouldn't want to use relative imports for cross-package imports though.