Add ability to query number of buffered bytes available on buffered I/O

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-01-01.08:31:45.113>
labels = ['type-feature', 'library', 'expert-IO']
title = 'Add ability to query number of buffered bytes available on buffered I/O'
updated_at = <Date 2018-06-27.04:01:30.244>
user = 'https://github.com/kata198'

bugs.python.org fields:

activity = <Date 2018-06-27.04:01:30.244>
actor = 'josnyder'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'IO']
creation = <Date 2018-01-01.08:31:45.113>
creator = 'kata198'
dependencies = []
files = ['47359']
hgrepos = []
issue_num = 32475
keywords = ['patch']
message_count = 4.0
messages = ['309328', '309334', '310035', '320542']
nosy_count = 4.0
nosy_names = ['pitrou', 'martin.panter', 'kata198', 'josnyder']
pr_nums = ['7947']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32475'
versions = ['Python 3.6']