email MIME splitting

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-04.12:48:46.668>
labels = ['type-security', '3.8', 'expert-email', '3.10', '3.7', '3.9']
title = 'email MIME splitting'
updated_at = <Date 2021-04-12.16:50:21.791>
user = 'https://bugs.python.org/martinortner'

bugs.python.org fields:

activity = <Date 2021-04-12.16:50:21.791>
actor = 'trrhodes'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2021-02-04.12:48:46.668>
creator = 'martin.ortner'
dependencies = []
files = []
hgrepos = []
issue_num = 43123
keywords = ['patch']
message_count = 3.0
messages = ['386479', '386589', '390867']
nosy_count = 4.0
nosy_names = ['barry', 'r.david.murray', 'trrhodes', 'martin.ortner']
pr_nums = ['24475']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'security'
url = 'https://bugs.python.org/issue43123'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']