[RFE] tarfile: add an option to change the "blocking factor"

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-10-12.14:17:19.775>
labels = ['3.8', 'type-feature', 'library']
title = '[RFE] tarfile: add an option to change the "blocking factor"'
updated_at = <Date 2018-06-20.13:04:55.361>
user = 'https://github.com/stratakis'

bugs.python.org fields:

activity = <Date 2018-06-20.13:04:55.361>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-10-12.14:17:19.775>
creator = 'cstratak'
dependencies = []
files = ['47218']
hgrepos = []
issue_num = 31774
keywords = []
message_count = 4.0
messages = ['304241', '304457', '304458', '320075']
nosy_count = 4.0
nosy_names = ['lars.gustaebel', 'vstinner', 'cstratak', 'nitishch']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31774'
versions = ['Python 3.8']