classicupgrade: fix a a bytes-like object is required, not 'str' error

classicupgrade: fix a a bytes-like object is required, not 'str' error

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14136

Signed-off-by: Bjoern Jacke <bjacke@samba.org>
Reviewed-by: Björn Baumbach <bb@samba.org>
Autobuild-User(master): Björn Jacke <bjacke@samba.org>
Autobuild-Date(master): Mon Sep 23 12:58:20 UTC 2019 on sn-devel-184

(cherry picked from commit 465e518d6cc200eefa38643e720ce64e53abac2e)

Autobuild-User(v4-10-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-10-test): Tue Sep 24 20:46:20 UTC 2019 on sn-devel-144