urljoin since 3.5 incorrectly filters out double slashes

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-05-11.11:53:50.366>
labels = ['3.7', '3.8', 'type-bug', 'library', '3.9']
title = 'urljoin since 3.5 incorrectly filters out double slashes'
updated_at = <Date 2021-12-01.23:04:01.306>
user = 'https://bugs.python.org/DavidBell'

bugs.python.org fields:

activity = <Date 2021-12-01.23:04:01.306>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-05-11.11:53:50.366>
creator = 'David Bell'
dependencies = []
files = []
hgrepos = []
issue_num = 40594
keywords = []
message_count = 9.0
messages = ['368623', '370821', '371092', '371098', '371101', '371135', '371685', '371691', '407503']
nosy_count = 6.0
nosy_names = ['orsenthil', 'taleinat', 'op368', 'Ido Michael', 'David Bell', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40594'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']