Use python3 in shebang lines for util scripts

Some distributions (notably, debian bookworm) have deprecated the
`python` interpreter in favor of `python3`. Since our scripts are
python3 anyway, use the proper numbered version in shebang to make
scripts easily executable.

(cherry picked from commit 480dcdef9a)

bin/tests/convert-trs-to-junit.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 #

bin/tests/system/doth/get_openssl_version.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 #

bin/tests/system/doth/stress_http_quota.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 #

contrib/gitchangelog/gitchangelog.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 ############################################################################
 # Copyright (c) 2018, Valentin Lab
 # All rights reserved.