expose the offset of a zipfile from the start of the file as a public API

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-04-16.13:59:27.010>
labels = ['type-feature', 'library', '3.9']
title = 'zipfile module: new feature (two lines of code), useful for test, security and forensics'
updated_at = <Date 2020-04-18.14:35:44.170>
user = 'https://github.com/massimosala'

bugs.python.org fields:

activity = <Date 2020-04-18.14:35:44.170>
actor = 'massimosala'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-04-16.13:59:27.010>
creator = 'massimosala'
dependencies = []
files = ['49067']
hgrepos = []
issue_num = 40301
keywords = ['patch']
message_count = 13.0
messages = ['366597', '366601', '366685', '366689', '366690', '366695', '366709', '366711', '366712', '366713', '366714', '366717', '366720']
nosy_count = 6.0
nosy_names = ['twouters', 'alanmcintyre', 'steven.daprano', 'serhiy.storchaka', 'dhillier', 'massimosala']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40301'
versions = ['Python 3.9']