[issue41900] XML C14N serialisation fails with default namespace

[Stefan Behnel]

New submission from Stefan Behnel <stefan_ml at behnel.de>:

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

----------
components: XML
messages: 377743
nosy: scoder
priority: normal
severity: normal
stage: needs patch
status: open
title: XML C14N serialisation fails with default namespace
type: behavior
versions: Python 3.10, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41900>
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