more portable python shebangs (#5816)

This is useful on systems like NixOS, where python3 is not in
/usr/bin/python3 as well as for people using alternative ways to
install python such as virtualenv/pyenv.
diff --git a/test/test-exec-deserialization.py b/test/test-exec-deserialization.py
index b974b1c..39a9e62 100755
--- a/test/test-exec-deserialization.py
+++ b/test/test-exec-deserialization.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 #
 #  Copyright 2017 Michal Sekletar <msekleta@redhat.com>