tarfile: incorrectly treats regular file as directory

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-09-22.21:42:01.827>
labels = ['3.7', 'type-bug', 'library']
title = 'tarfile: incorrectly treats regular file as directory'
updated_at = <Date 2017-10-04.17:21:19.906>
user = 'https://bugs.python.org/JoeTsai'

bugs.python.org fields:

activity = <Date 2017-10-04.17:21:19.906>
actor = 'Joe Tsai'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-09-22.21:42:01.827>
creator = 'Joe Tsai'
dependencies = []
files = ['47188']
hgrepos = []
issue_num = 31557
keywords = []
message_count = 5.0
messages = ['302778', '303431', '303655', '303676', '303715']
nosy_count = 4.0
nosy_names = ['lars.gustaebel', 'serhiy.storchaka', 'nitishch', 'Joe Tsai']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31557'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7']