XML C14N serialisation fails with default namespace

[scoder]

BPO	41900
Nosy	@scoder, @miss-islington
PRs	bpo-41900: C14N 2.0 serialisation failed for unprefixed attributes when a default namespace was defined. #22474 [3.9] bpo-41900: C14N 2.0 serialisation failed for unprefixed attributes when a default namespace was defined. (GH-22474) #22507 [3.8] bpo-41900: C14N 2.0 serialisation failed for unprefixed attributes when a default namespace was defined. (GH-22474) #22508

^{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 2020-10-03.06:44:34.583>
created_at = <Date 2020-10-01.08:09:31.705>
labels = ['expert-XML', 'type-bug', '3.8', '3.9', '3.10']
title = 'XML C14N serialisation fails with default namespace'
updated_at = <Date 2020-10-03.06:44:34.583>
user = 'https://github.com/scoder'

bugs.python.org fields:

activity = <Date 2020-10-03.06:44:34.583>
actor = 'scoder'
assignee = 'none'
closed = True
closed_date = <Date 2020-10-03.06:44:34.583>
closer = 'scoder'
components = ['XML']
creation = <Date 2020-10-01.08:09:31.705>
creator = 'scoder'
dependencies = []
files = []
hgrepos = []
issue_num = 41900
keywords = ['patch']
message_count = 4.0
messages = ['377743', '377855', '377856', '377857']
nosy_count = 2.0
nosy_names = ['scoder', 'miss-islington']
pr_nums = ['22474', '22507', '22508']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41900'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[scoder]

import xml.etree.ElementTree as ET
xml="""
<X xmlns="http://soap.sforce.com/2006/04/metadata">
    <Y targets="lightning__AppPage,lightning__HomePage"></Y>
</X>"""
print(ET.canonicalize(xml))

Fails with:
ValueError: Namespace "" is not declared in scope
when trying to build the QName of the unnamespaced "targets" attribute.

Originally reported for lxml here:
https://bugs.launchpad.net/lxml/+bug/1869455

[scoder]

New changeset 6a412c9 by scoder in branch 'master':
bpo-41900: C14N 2.0 serialisation failed for unprefixed attributes when a default namespace was defined. (GH-22474)
6a412c9

[scoder]

New changeset a0f2b66 by Miss Skeleton (bot) in branch '3.9':
bpo-41900: C14N 2.0 serialisation failed for unprefixed attributes when a default namespace was defined. (GH-22474) (GH-22507)
a0f2b66

[scoder]

New changeset cfed534 by Miss Skeleton (bot) in branch '3.8':
bpo-41900: C14N 2.0 serialisation failed for unprefixed attributes when a default namespace was defined. (GH-22474) (GH-22508)
cfed534