GzipFile's .seekable() returns True even if underlying buffer is not seekable

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-03-28.16:28:47.984>
labels = ['3.8', 'library']
title = "GzipFile's .seekable() returns True even if underlying buffer is not seekable"
updated_at = <Date 2018-11-09.08:55:32.150>
user = 'https://github.com/waltaskew'

bugs.python.org fields:

activity = <Date 2018-11-09.08:55:32.150>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-03-28.16:28:47.984>
creator = 'Walt Askew'
dependencies = []
files = []
hgrepos = []
issue_num = 33173
keywords = ['patch']
message_count = 5.0
messages = ['314613', '327206', '327226', '329505', '329506']
nosy_count = 6.0
nosy_names = ['pitrou', 'martin.panter', 'serhiy.storchaka', 'cheryl.sabella', 'Walt Askew', 'xtreak']
pr_nums = ['6303']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue33173'
versions = ['Python 3.8']