python: Add __future__ imports for python 3 compatibility

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

Closes ticket 32732.

scripts/codegen/fuzzing_include_am.py

@@ -1,5 +1,10 @@
 #!/usr/bin/python
 
+# Future imports for Python 2.7, mandatory in 3.0
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
 FUZZERS = """
 	consensus
 	descriptor