python: Add __future__ imports for python 3 compatibility

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

Closes ticket 32732.

scripts/codegen/gen_server_ciphers.py

@@ -8,6 +8,11 @@
 #
 # Run it on all the files in your openssl include directory.
 
+# 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 re
 import sys