asyncio child watchers aren't fork friendly

[1st1]

BPO	33688
Nosy	@asvetlov, @1st1

^{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 = None
created_at = <Date 2018-05-29.22:45:03.558>
labels = ['3.7', '3.8', 'expert-asyncio']
title = "asyncio child watchers aren't fork friendly"
updated_at = <Date 2018-05-29.22:56:08.480>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2018-05-29.22:56:08.480>
actor = 'zmedico'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['asyncio']
creation = <Date 2018-05-29.22:45:03.558>
creator = 'yselivanov'
dependencies = []
files = []
hgrepos = []
issue_num = 33688
keywords = []
message_count = 1.0
messages = ['318133']
nosy_count = 3.0
nosy_names = ['zmedico', 'asvetlov', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue33688'
versions = ['Python 3.7', 'Python 3.8']

[1st1]

_UnixDefaultEventLoopPolicy doesn't detect fork and tries to re-use the inherited child watcher. [1] is my idea how to fix this.

Andrew, AFAIK subprocesses was something that you worked on at some point. Do you have time to look into this? Need tests and I'm not sure if I got [1] quite right.

This probably won't make it to 3.7.0, but 3.7.1 seems like a good target.

[1] 1st1@b4e6b26

[kumaraditya303]

This is included in #66285