setup.py: fix error message when running with python-2.3

When setup.py tries to check the python version, the check actually
won't give a usable error message but it'll raise a SyntaxError. Fix
this by not using generator expressions.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Karl Hasselström <kha@treskal.com>