open doesn't call IncrementalEncoder with final=True

GitHub fields:

assignee = 'https://github.com/malemburg'
closed_at = None
created_at = <Date 2018-12-29.06:00:15.132>
labels = ['type-bug', '3.7', 'expert-IO']
title = "open doesn't call IncrementalEncoder with final=True"
updated_at = <Date 2019-01-04.22:12:34.613>
user = 'https://github.com/haney'

bugs.python.org fields:

activity = <Date 2019-01-04.22:12:34.613>
actor = 'vstinner'
assignee = 'lemburg'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2018-12-29.06:00:15.132>
creator = 'haney'
dependencies = []
files = ['48022']
hgrepos = []
issue_num = 35611
keywords = []
message_count = 2.0
messages = ['332701', '332740']
nosy_count = 5.0
nosy_names = ['lemburg', 'doerwalter', 'vstinner', 'martin.panter', 'haney']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35611'
versions = ['Python 3.7']