Reverted "Fixed #27878, refs #23919 -- Used python3 shebangs."

This reverts commit 4cffa9a1ff.

tests/i18n/sampleproject/manage.py

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

tests/i18n/sampleproject/update_catalogs.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 
 """
 Helper script to update sampleproject's translation catalogs.

tests/runtests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 import argparse
 import atexit
 import copy