Allow inspecting buffering attribute of IO objects

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-05-29.08:59:06.599>
labels = ['type-feature', '3.7', 'expert-IO']
title = 'Allow inspecting buffering attribute of IO objects'
updated_at = <Date 2017-05-31.16:13:25.672>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2017-05-31.16:13:25.672>
actor = 'pitrou'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2017-05-29.08:59:06.599>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 30504
keywords = []
message_count = 3.0
messages = ['294683', '294685', '294849']
nosy_count = 4.0
nosy_names = ['pitrou', 'benjamin.peterson', 'stutzbach', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30504'
versions = ['Python 3.7']