test: use 'env python3'

'env python' only works on some RH derivatives, on Debian and
derivatives it does not work by default.

Fixes #10

Author: bluca

tests/native-messaging-hosts/write-on-close.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 import sys
 import os
 from pathlib import Path