Constructor signature is duplicated in the help of namedtuples

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-07-18.11:04:22.806>
labels = ['3.7', 'type-bug', 'library']
title = 'Constructor signature is duplicated in the help of namedtuples'
updated_at = <Date 2017-07-21.08:54:23.446>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2017-07-21.08:54:23.446>
actor = 'SylvainDe'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-07-18.11:04:22.806>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 30959
keywords = []
message_count = 3.0
messages = ['298582', '298766', '298784']
nosy_count = 4.0
nosy_names = ['rhettinger', 'ncoghlan', 'serhiy.storchaka', 'SylvainDe']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30959'
versions = ['Python 3.7']