Fix false positives in circular import detection with from-imports

[pitrou]

BPO	43517
Nosy	@brettcannon, @gpshead, @ncoghlan, @pitrou, @ericsnowcurrently, @serhiy-storchaka, @miss-islington, @nanjekyejoannah, @cebtenzzre
PRs	bpo-43517: Fix false positive in detection of circular imports #24895 [3.9] bpo-43517: Fix false positive in detection of circular imports (GH-24895) #24948 bpo-43517 (followup): Install additional test directory #24950 [3.9] bpo-43517 (followup): Install additional test directory (GH-24950) #24951

^{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 = 'https://github.com/pitrou'
closed_at = <Date 2021-03-20.21:52:36.447>
created_at = <Date 2021-03-16.16:07:50.206>
labels = ['deferred-blocker', 'type-bug', 'library', '3.9', '3.10']
title = 'Fix false positives in circular import detection with from-imports'
updated_at = <Date 2021-03-20.21:52:36.447>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2021-03-20.21:52:36.447>
actor = 'pitrou'
assignee = 'pitrou'
closed = True
closed_date = <Date 2021-03-20.21:52:36.447>
closer = 'pitrou'
components = ['Library (Lib)']
creation = <Date 2021-03-16.16:07:50.206>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 43517
keywords = ['patch']
message_count = 6.0
messages = ['388854', '389172', '389174', '389179', '389183', '389185']
nosy_count = 9.0
nosy_names = ['brett.cannon', 'gregory.p.smith', 'ncoghlan', 'pitrou', 'eric.snow', 'serhiy.storchaka', 'miss-islington', 'nanjekyejoannah', 'cebtenzzre']
pr_nums = ['24895', '24948', '24950', '24951']
priority = 'deferred blocker'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43517'
versions = ['Python 3.9', 'Python 3.10']

[pitrou]

This seems to be caused by a logic error in the patch for bpo-35943.

It has been causing multiple issues for multi-threaded and multi-process systems written in Python:

• bpo-41567
• bpo-43515
• Python 3.9 compatibility dask/distributed#4168

[pitrou]

New changeset 2fd16ef by Antoine Pitrou in branch 'master':
bpo-43517: Fix false positive in detection of circular imports (bpo-24895)
2fd16ef

[miss-islington]

New changeset ac17ed6 by Antoine Pitrou in branch '3.9':
[3.9] bpo-43517: Fix false positive in detection of circular imports (GH-24895) (GH-24948)
ac17ed6