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

django/bin/django-admin.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 from django.core import management
 
 if __name__ == "__main__":

django/conf/project_template/manage.py-tpl

-#!/usr/bin/env python
+#!/usr/bin/env python3
 import os
 import sys
 

django/contrib/admin/bin/compress.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 import argparse
 import os
 import subprocess

docs/topics/testing/advanced.txt

@@ -333,7 +333,7 @@ Let's take a look inside a couple of those files:
 .. snippet::
     :filename: runtests.py
 
-    #!/usr/bin/env python
+    #!/usr/bin/env python3
     import os
     import sys
 

scripts/manage_translations.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # This python file contains utility scripts to manage Django translations.
 # It has to be run inside the django git root directory.

tests/i18n/sampleproject/manage.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 import os
 import sys
 

tests/i18n/sampleproject/update_catalogs.py

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

tests/runtests.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 import argparse
 import atexit
 import copy