python: convert shebangs to python3

This is the last bit of "python" left in the notmuch codebase.

https://www.python.org/dev/peps/pep-0394/#recommendation encourages
"third-party distributors" to use more-specific shebang lines.  I'm
not certain that the notmuch project itself is a "third-party
contributor" but I think this is a safe way to encourage people to use
python3 when they're developing notmuch.

We already have python3 explicitly elsewhere in the codebase for
developers (in nmbug).

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

bindings/python/setup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """
 This file is part of notmuch.

test/json_check_nodes.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 import re
 import sys
 import json