Add minimum required Python version (2.5)

Well, #9800 doesn't give the exact recipe, but it is contained in the setup.py for Trac:

min_python = (2, 5)
if sys.version_info < min_python:
    print "Trac requires Python %d.%d or later" % min_python
    sys.exit(1)
if sys.version_info >= (3,):
    print "Trac doesn't support Python 3 (yet)"
    sys.exit(1)

So you could just copy that into your setup.py and change the printed message.

(In [12057]) Fixes #9963, Refs #9800, #8276, #8803, #9146: Added check in setup.py for minimum required Python version. Python 2.5 is currently required. Later, we'll aim to restore full Python 2.4 compatibility.