Update the stdlib to fall back to __spec__.loader if __loader__ isn't defined

[brettcannon]

BPO	42133
Nosy	@brettcannon
PRs	bpo-42133: update parts of the stdlib to fall back to __spec__.loader when __loader__ is missing #22929

^{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 2020-11-09.21:18:14.847>
created_at = <Date 2020-10-23.22:46:40.277>
labels = ['type-bug', 'library', '3.10']
title = "Update the stdlib to fall back to __spec__.loader if __loader__ isn't defined"
updated_at = <Date 2020-11-09.21:18:14.846>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2020-11-09.21:18:14.846>
actor = 'brett.cannon'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-09.21:18:14.847>
closer = 'brett.cannon'
components = ['Library (Lib)']
creation = <Date 2020-10-23.22:46:40.277>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 42133
keywords = ['patch']
message_count = 2.0
messages = ['379486', '380491']
nosy_count = 1.0
nosy_names = ['brett.cannon']
pr_nums = ['22929']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42133'
versions = ['Python 3.10']

[brettcannon]

By making sure the stdlib can handle the case where __loader__ isn't defined but __spec__.parent is, eventually the former could be dropped for the latter.

[brettcannon]

New changeset 825ac38 by Brett Cannon in branch 'master':
bpo-42133: update parts of the stdlib to fall back to __spec__.loader when __loader__ is missing (bpo-22929)
825ac38