Remove use of securesystemslib.util.TempFile
Remove use of securesystemslib.util.TempFile

So that we can remove securesystemslib.util.TempFile switch to using
a combination of tempfile.TemporaryFile() and
securesystemslib.util.persist_temp_file()

Signed-off-by: Joshua Lock <edc229f50a11a11cdece0270af6320fb299dca1e@vmware.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib