tarfile: ignore_zeros = True exceedingly slow on a sparse tar file

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-06-02.15:18:22.457>
labels = ['3.7']
title = 'tarfile: ignore_zeros = True exceedingly slow on a sparse tar file'
updated_at = <Date 2020-06-02.15:18:22.457>
user = 'https://bugs.python.org/mxmlnkn'

bugs.python.org fields:

activity = <Date 2020-06-02.15:18:22.457>
actor = 'mxmlnkn'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2020-06-02.15:18:22.457>
creator = 'mxmlnkn'
dependencies = []
files = []
hgrepos = []
issue_num = 40843
keywords = []
message_count = 1.0
messages = ['370611']
nosy_count = 1.0
nosy_names = ['mxmlnkn']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40843'
versions = ['Python 3.7']