tarfile.TarFile may write corrupt files if not closed

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-09-13.17:20:55.522>
labels = ['3.7', 'type-bug', 'library']
title = 'tarfile.TarFile may write corrupt files if not closed'
updated_at = <Date 2018-09-13.21:47:05.383>
user = 'https://github.com/lordmauve'

bugs.python.org fields:

activity = <Date 2018-09-13.21:47:05.383>
actor = 'ned.deily'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-09-13.17:20:55.522>
creator = 'lordmauve'
dependencies = []
files = []
hgrepos = []
issue_num = 34662
keywords = []
message_count = 2.0
messages = ['325266', '325269']
nosy_count = 2.0
nosy_names = ['lars.gustaebel', 'lordmauve']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34662'
versions = ['Python 3.7']