urlencode should accept generator as values for mappings when doseq=True

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-10-05.20:12:52.109>
labels = ['3.8', 'type-feature', 'library']
title = 'urlencode should accept generator as values for mappings when doseq=True'
updated_at = <Date 2019-08-25.11:04:53.916>
user = 'https://github.com/francoisfreitag'

bugs.python.org fields:

activity = <Date 2019-08-25.11:04:53.916>
actor = 'taleinat'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-10-05.20:12:52.109>
creator = 'francois.freitag'
dependencies = []
files = []
hgrepos = []
issue_num = 31706
keywords = ['patch']
message_count = 4.0
messages = ['303784', '303806', '306908', '350449']
nosy_count = 4.0
nosy_names = ['orsenthil', 'taleinat', 'serhiy.storchaka', 'francois.freitag']
pr_nums = ['3901']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31706'
versions = ['Python 3.8']