setup.py: drop compatibility with Python 2.4 and 2.5 (BC)

The last blocker for dropping Python 2.4 was Centos 5. We now provide our own
Mercurial package for Centos 5 with a bundled Python2.7.

I'm therefore happy to officially drop compatibility with Python 2.4 (and
Python 2.5 that nobody really cares about). This open the season for code
cleanup.

It is war's prize to take all vantage.

setup.py

@@ -5,8 +5,8 b''
 # 'python setup.py --help' for more options
 
 import sys, platform
-if getattr(sys, 'version_info', (0, 0, 0)) < (2, 4, 0, 'final'):
-    raise SystemExit("Mercurial requires Python 2.4 or later.")
+if getattr(sys, 'version_info', (0, 0, 0)) < (2, 6, 0, 'final'):
+    raise SystemExit("Mercurial requires Python 2.6 or later.")
 
 if sys.version_info[0] >= 3:
     def b(s):