DOC/MAINT: Python 2.6 or 3.2 required

Installation docs still stated Python 2.4 is supported, while not mentioning
3.x at all. Also corrected the statement that distutils is in python-dev on
Debian, while it's actually in the main package.

Several parts of the dev docs and some comments also referenced now-irrelevant
2.5 bugs/limitations.
Merge pull request #5120 from larsmans/install-docs

DOC: update installation docs