urllib2 fails for proxy credentials that contains a '/' character

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-08-26.01:24:24.624>
labels = ['type-bug', 'build']
title = "urllib2 fails for proxy credentials that contains a '/' character"
updated_at = <Date 2018-09-03.08:59:47.583>
user = 'https://bugs.python.org/Balibax5445'

bugs.python.org fields:

activity = <Date 2018-09-03.08:59:47.583>
actor = 'xtreak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Build']
creation = <Date 2018-08-26.01:24:24.624>
creator = 'Balibax5445'
dependencies = []
files = []
hgrepos = []
issue_num = 34505
keywords = []
message_count = 2.0
messages = ['324111', '324503']
nosy_count = 2.0
nosy_names = ['xtreak', 'Balibax5445']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34505'
versions = []