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.

test/create-sys-script.py

@@ -1,6 +1,6 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
-OUTFILE_HEADER = """#!/usr/bin/python3
+OUTFILE_HEADER = """#!/usr/bin/env python3
 #
 # create-sys-script.py
 #