scons: bump required python version to 2.7 to support pybind11

Change-Id: Ic3652f975477f2e5d144e054489ab73ed9f82b55
Reviewed-on: https://gem5-review.googlesource.com/4440
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Joe Gross <joe.gross@amd.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

SConstruct

@@ -93,10 +93,9 @@ For more details, see:
 """
     raise
 
-# We ensure the python version early because because python-config
-# requires python 2.5
+# pybind11 requires python 2.7
 try:
-    EnsurePythonVersion(2, 5)
+    EnsurePythonVersion(2, 7)
 except SystemExit, e:
     print """
 You can use a non-default installation of the Python interpreter by