MIMEMultipart enforces line endings also for binary subparts

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-12-22.16:36:56.167>
labels = ['type-bug', 'expert-email', '3.9']
title = 'MIMEMultipart enforces line endings also for binary subparts'
updated_at = <Date 2021-12-22.16:36:56.167>
user = 'https://bugs.python.org/sophonet'

bugs.python.org fields:

activity = <Date 2021-12-22.16:36:56.167>
actor = 'sophonet'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2021-12-22.16:36:56.167>
creator = 'sophonet'
dependencies = []
files = []
hgrepos = []
issue_num = 46154
keywords = []
message_count = 1.0
messages = ['409041']
nosy_count = 3.0
nosy_names = ['barry', 'r.david.murray', 'sophonet']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46154'
versions = ['Python 3.9']