Change bpttool Python shebang line to use /usr/bin/env

Also check that Python version is at least 2.6 since bpttool uses modern
exception syntax.

Bug: 72837107
Test: m droid
Change-Id: I3f282e0314037be281641b99675a41b9bc245d73
diff --git a/bpttool b/bpttool
index ae4bc8e..3a4b269 100755
--- a/bpttool
+++ b/bpttool
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 # Copyright 2016, The Android Open Source Project
 #
@@ -28,6 +28,10 @@
 import uuid
 import zlib
 
+# Python 2.6 required for modern exception syntax
+if sys.hexversion < 0x02060000:
+  print >> sys.stderr, "Python 2.6 or newer is required."
+  sys.exit(1)
 
 # Keywords used in JSON files.
 JSON_KEYWORD_SETTINGS = 'settings'