Prohibit Python3 in Python version check. (#1872)

bin/spack

@@ -25,9 +25,9 @@
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 import sys
-if not sys.version_info[:2] >= (2, 6):
+if (sys.version_info[0] > 2) or (sys.version_info[:2] < (2, 6)):
     v_info = sys.version_info[:3]
-    sys.exit("Spack requires Python 2.6 or higher. "
+    sys.exit("Spack requires Python 2.6 or 2.7. "
              "This is Python %d.%d.%d." % v_info)
 
 import os