Make the whole code of the Python 2.7 version use even more of Python 2.7's features

[lebigot]

The Python 2.7 code is mostly based on the last Python 2.6 and even Python 2.5 version. Some details of the code could be updated so as to take advantage of the features that Python 2.7's brought, or so as to be adapted to Python 2.7 directly (parts of the code handle differences between Python 2.6 and Python 2.7, for instance).

Examples:

• Dictionary comprehension,
• Sets literals,
• String formatting with {}.

The code tends to contain comments like Python 2.5 or Python 2.6 or Python 2.7+ in places where the code could be upgraded.

[lebigot]

Was this already done when the code base moved to Python 3?