email.contentmanager should use IANA encoding

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-08-22.09:00:20.174>
labels = ['3.7', 'expert-email']
title = 'email.contentmanager should use IANA encoding'
updated_at = <Date 2018-08-22.09:18:02.042>
user = 'https://bugs.python.org/era'

bugs.python.org fields:

activity = <Date 2018-08-22.09:18:02.042>
actor = 'era'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2018-08-22.09:00:20.174>
creator = 'era'
dependencies = []
files = []
hgrepos = []
issue_num = 34459
keywords = []
message_count = 2.0
messages = ['323869', '323871']
nosy_count = 3.0
nosy_names = ['barry', 'r.david.murray', 'era']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue34459'
versions = ['Python 3.7']