asyncio: how to set a "Proactor event loop" policy? Issue with asyncio.run()

BPO	33792
Nosy	@vstinner, @asvetlov, @1st1, @miss-islington
PRs	bpo-33792: Add selector and proactor windows policies #7487 [3.7] bpo-33792: Add selector and proactor windows policies (GH-7487) #7508

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-06-08.03:43:15.087>
created_at = <Date 2018-06-07.10:02:04.335>
labels = ['3.7', '3.8', 'expert-asyncio']
title = 'asyncio: how to set a "Proactor event loop" policy? Issue with asyncio.run()'
updated_at = <Date 2018-06-08.03:43:15.086>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-06-08.03:43:15.086>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2018-06-08.03:43:15.087>
closer = 'yselivanov'
components = ['asyncio']
creation = <Date 2018-06-07.10:02:04.335>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 33792
keywords = ['patch']
message_count = 5.0
messages = ['318919', '318924', '318929', '319002', '319004']
nosy_count = 4.0
nosy_names = ['vstinner', 'asvetlov', 'yselivanov', 'miss-islington']
pr_nums = ['7487', '7508']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue33792'
versions = ['Python 3.7', 'Python 3.8']