selectors.KqueueSelector should not be the default selector

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-04-10.23:23:51.928>
labels = ['3.8', '3.9', 'expert-IO', '3.7', 'library', 'expert-asyncio']
title = 'selectors.KqueueSelector should not be the default selector'
updated_at = <Date 2020-04-10.23:23:51.928>
user = 'https://github.com/russelldavis'

bugs.python.org fields:

activity = <Date 2020-04-10.23:23:51.928>
actor = 'russelldavis'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'IO', 'asyncio']
creation = <Date 2020-04-10.23:23:51.928>
creator = 'russelldavis'
dependencies = []
files = []
hgrepos = []
issue_num = 40252
keywords = []
message_count = 1.0
messages = ['366168']
nosy_count = 3.0
nosy_names = ['asvetlov', 'yselivanov', 'russelldavis']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40252'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']