Allow paths to be joined without worrying about a leading slash

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-06-18.12:58:26.952>
labels = ['type-bug', 'library', '3.11']
title = 'Allow paths to be joined without worrying about a leading slash'
updated_at = <Date 2021-11-26.21:02:59.697>
user = 'https://bugs.python.org/zbysz'

bugs.python.org fields:

activity = <Date 2021-11-26.21:02:59.697>
actor = 'eryksun'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-06-18.12:58:26.952>
creator = 'zbysz'
dependencies = []
files = []
hgrepos = []
issue_num = 44452
keywords = []
message_count = 10.0
messages = ['396058', '396449', '396562', '396566', '396571', '396588', '396591', '401563', '407076', '407094']
nosy_count = 7.0
nosy_names = ['eric.smith', 'zbysz', 'serhiy.storchaka', 'eryksun', 'veky', 'smarie', 'lutecki']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44452'
versions = ['Python 3.11']