mailbox: Implement >From_ decoding on input from mbox

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-12-23.14:16:48.385>
labels = ['type-feature', 'expert-email']
title = 'mailbox: Implement >From_ decoding on input from mbox'
updated_at = <Date 2017-01-03.12:21:51.618>
user = 'https://bugs.python.org/bpoaugust'

bugs.python.org fields:

activity = <Date 2017-01-03.12:21:51.618>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2016-12-23.14:16:48.385>
creator = 'bpoaugust'
dependencies = []
files = ['46017']
hgrepos = []
issue_num = 29053
keywords = []
message_count = 5.0
messages = ['283879', '283882', '283883', '283914', '284500']
nosy_count = 3.0
nosy_names = ['barry', 'r.david.murray', 'bpoaugust']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue29053'
versions = ['Python 3.5']