mailbox: It would be nice to move mailbox.Message from legacy email.message.Message API to new EmailMessage API

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-03-01.05:44:18.590>
labels = ['type-feature', '3.8', 'expert-email']
title = 'mailbox: It would be nice to move mailbox.Message from legacy email.message.Message API to new EmailMessage API'
updated_at = <Date 2018-03-02.02:29:36.775>
user = 'https://github.com/maxking'

bugs.python.org fields:

activity = <Date 2018-03-02.02:29:36.775>
actor = 'maxking'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2018-03-01.05:44:18.590>
creator = 'maxking'
dependencies = []
files = []
hgrepos = []
issue_num = 32975
keywords = []
message_count = 4.0
messages = ['313082', '313083', '313120', '313122']
nosy_count = 3.0
nosy_names = ['barry', 'r.david.murray', 'maxking']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32975'
versions = ['Python 3.8']