smtplib: allow custom policy or use msg.policy in send_message

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-03-17.16:48:39.898>
labels = ['3.8', 'expert-email', '3.10', '3.11', '3.7', 'library', '3.9']
title = 'smtplib: allow custom policy or use msg.policy in send_message'
updated_at = <Date 2022-03-17.18:34:59.131>
user = 'https://github.com/Miksus'

bugs.python.org fields:

activity = <Date 2022-03-17.18:34:59.131>
actor = 'Miksus'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'email']
creation = <Date 2022-03-17.16:48:39.898>
creator = 'Miksus'
dependencies = []
files = []
hgrepos = []
issue_num = 47047
keywords = []
message_count = 2.0
messages = ['415428', '415436']
nosy_count = 3.0
nosy_names = ['barry', 'r.david.murray', 'Miksus']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue47047'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']