python: Add __future__ imports for python 3 compatibility

Except for src/ext, which we may not want to modify.

Closes ticket 32732.

scripts/maint/practracker/util.py

@@ -1,3 +1,8 @@
+# Future imports for Python 2.7, mandatory in 3.0
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
 import os
 
 # We don't want to run metrics for unittests, automatically-generated C files,