Big Python coding style cleanup

This is a huge set of changes (10 000 lines), most of which are automated. Thus, you do not need to review dfa21e70 (courtesy of autopep8), nor the part of 0449d71d that deals with convertrules.py (a bare search-and-replace). On the other hand, I would appreciate feedback on all the TODO comments added as part of c1b922cb.

This came after this thread. While I am aware that rewriting shortly more than half the lines of our Python scripts will hamper the usability of git-blame, I cannot think of a better solution to adress the coding style concerns that arose in !212 (closed).

Instances of str as variable name were converted to s as suggested by Carl.