Switch to using six (#1189)

With the Python 2.7 deadline fast approaching, modernize the codebase
making use of the modernize module to switch to using six, as well as
other upcoming features, such as absolute imports . Stop using 2to3
for Travis, yay!

scripts/add_attribute.py

@@ -28,6 +28,7 @@
 #                                                                              #
 ################################################################################
 
+from __future__ import absolute_import
 import sys
 import os.path
 

scripts/fix_headers.py

@@ -26,6 +26,8 @@
 #                                                                              #
 ################################################################################
 
+from __future__ import absolute_import
+from __future__ import print_function
 import os
 import subprocess
 
@@ -153,17 +155,17 @@ def findHeadersAndFiles():
             elif fullname.endswith(".pyc"):
                 pass
             else:
-                print "Don't know what to do with", filename
+                print("Don't know what to do with", filename)
 
 
 def main():
     for header, filename in findHeadersAndFiles():
-        print "Analyzing", filename
+        print("Analyzing", filename)
         with open(filename) as f:
             lines = list(line.rstrip() for line in f)
         newLines = header.fix(filename, lines)
         if newLines != lines:
-            print " => actually modifying", filename
+            print(" => actually modifying", filename)
             with open(filename, "w") as f:
                 for line in newLines:
                     f.write(line + "\n")