From, To, Cc lines break when calling send_message()

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-05-01.11:51:56.273>
labels = ['type-bug', 'expert-email']
title = 'From, To, Cc lines break when calling send_message()'
updated_at = <Date 2018-07-31.13:01:46.476>
user = 'https://github.com/jenstroeger'

bugs.python.org fields:

activity = <Date 2018-07-31.13:01:46.476>
actor = '_savage'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2018-05-01.11:51:56.273>
creator = '_savage'
dependencies = []
files = []
hgrepos = []
issue_num = 33398
keywords = []
message_count = 2.0
messages = ['315994', '322769']
nosy_count = 3.0
nosy_names = ['barry', 'r.david.murray', '_savage']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33398'
versions = ['Python 3.6']