setup.py: check version of Python

setup.py

@@ -11,6 +11,15 @@
 from __future__ import print_function
 
 import sys
+
+v = sys.version_info
+if v[:2] not in [(2, 7), (2, 6)]:
+    error = "This version of Kwant requires Python 2.6 or 2.7.\n"
+    if v[0] >= 2:
+        error += "Please use Kwant 1.2 or above."
+    print(error, file=sys.stderr)
+    sys.exit(1)
+
 import re
 import os
 import glob