urljoin behavior unclear/not following RFC 3986

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-06-11.15:45:18.972>
labels = ['3.7', 'type-bug', 'library']
title = 'urljoin behavior unclear/not following RFC 3986'
updated_at = <Date 2021-12-01.23:04:05.768>
user = 'https://bugs.python.org/matthewkenigsberg'

bugs.python.org fields:

activity = <Date 2021-12-01.23:04:05.768>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-06-11.15:45:18.972>
creator = 'matthewkenigsberg'
dependencies = []
files = []
hgrepos = []
issue_num = 37235
keywords = []
message_count = 2.0
messages = ['345243', '407504']
nosy_count = 4.0
nosy_names = ['orsenthil', 'xtreak', 'matthewkenigsberg', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37235'
versions = ['Python 3.7']