io.TextIOWrapper ignores silently partial write if buffer is unbuffered

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-07-06.17:33:53.629>
labels = ['type-bug', '3.8', 'expert-IO']
title = 'io.TextIOWrapper ignores silently partial write if buffer is unbuffered'
updated_at = <Date 2020-07-08.10:18:50.897>
user = 'https://github.com/manueljacob'

bugs.python.org fields:

activity = <Date 2020-07-08.10:18:50.897>
actor = 'eryksun'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2020-07-06.17:33:53.629>
creator = 'mjacob'
dependencies = []
files = []
hgrepos = []
issue_num = 41221
keywords = []
message_count = 8.0
messages = ['373151', '373167', '373203', '373207', '373216', '373287', '373289', '373300']
nosy_count = 6.0
nosy_names = ['pitrou', 'vstinner', 'methane', 'serhiy.storchaka', 'eryksun', 'mjacob']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41221'
versions = ['Python 3.8']