python:tests: Fix domain_backup test with Python 3.11

python:tests: Fix domain_backup test with Python 3.11

Traceback (most recent call last):
  File "bin/python/samba/tests/domain_backup.py", line 615, in test_backup_restore_with_conf
    self._test_backup_restore_with_conf()
  File "bin/python/samba/tests/domain_backup.py", line 244, in _test_backup_restore_with_conf
    self.restore_backup(backup_file, ["--configfile=" + smbconf])
  File "bin/python/samba/tests/domain_backup.py", line 421, in restore_backup
    self.run_cmd(args)
  File "bin/python/samba/tests/domain_backup.py", line 384, in run_cmd
    self.cleanup_tempdir()
  File "bin/python/samba/tests/domain_backup.py", line 370, in cleanup_tempdir
    shutil.rmtree(filepath)
  File "/usr/lib64/python3.11/shutil.py", line 732, in rmtree
    _rmtree_safe_fd(fd, path, onerror)
  File "/usr/lib64/python3.11/shutil.py", line 635, in _rmtree_safe_fd
    onerror(os.scandir, path, sys.exc_info())
  File "/usr/lib64/python3.11/shutil.py", line 631, in _rmtree_safe_fd
    with os.scandir(topfd) as scandir_it:
         ^^^^^^^^^^^^^^^^^
NotADirectoryError: [Errno 20] Not a directory:
  'st/tmp/tmp7usvex3t/samba-backup-2023-02-08T10-13-18.461113.tar.bz2'

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15308

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>