Untokenize does not round-trip ws before bs-nl

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-11-01.17:31:35.908>
labels = ['type-bug', '3.9']
title = 'Untokenize does not round-trip ws before bs-nl'
updated_at = <Date 2019-11-03.22:08:12.646>
user = 'https://github.com/edreamleo'

bugs.python.org fields:

activity = <Date 2019-11-03.22:08:12.646>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-11-01.17:31:35.908>
creator = 'edreamleo'
dependencies = []
files = []
hgrepos = []
issue_num = 38663
keywords = []
message_count = 5.0
messages = ['355827', '355898', '355899', '355900', '355910']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'edreamleo']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38663'
versions = ['Python 3.9']