Update shebangs to python3.

format-json

-#!/usr/bin/env python
+#!/usr/bin/env python3
 r"""Command-line tool to format software release JSON for slapos.
 
 Inspired by json.tool from python, but enforcing 2 spaces and non-sorted keys.

update-hash

-#!/usr/bin/env python
+#!/usr/bin/env python3
 """
 Helper script to update buildout.hash.cfg
 

validate-schema

-#!/usr/bin/env python
+#!/usr/bin/env python3
 import argparse
 import json
 import os.path