asyncio: selector_events: add_urgent() for urgent data to read, 3rd argument of select.select()

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-07-04.13:51:23.279>
labels = ['type-feature', '3.7', 'expert-asyncio']
title = 'asyncio: selector_events: add_urgent() for urgent data to read, 3rd argument of select.select()'
updated_at = <Date 2017-07-06.11:22:47.102>
user = 'https://github.com/pklanke'

bugs.python.org fields:

activity = <Date 2017-07-06.11:22:47.102>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['asyncio']
creation = <Date 2017-07-04.13:51:23.279>
creator = 'pklanke'
dependencies = []
files = []
hgrepos = []
issue_num = 30847
keywords = []
message_count = 9.0
messages = ['297652', '297653', '297654', '297655', '297722', '297765', '297809', '297821', '297823']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'vstinner', 'yselivanov', 'pklanke']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30847'
versions = ['Python 3.7']