Make assert_error compatible with Python 3.6
The re.Pattern class was introduced in Python 3.7. Use duck-typing to
distinguish between str and re pattern object.
Fixes: https://pagure.io/freeipa/issue/8179
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Sergey Orlov <sorlov@redhat.com>