gh-61290: Fix serializing attributes with the default_namespace option

[serhiy-storchaka]

Serialization with the default_namespace option failed for any attribute without a namespace:

>>> svg = ET.XML('<svg xmlns="http://www.w3.org/2000/svg"><rect fill="none"/></svg>')
>>> ET.tostring(svg, default_namespace='http://www.w3.org/2000/svg')
ValueError: cannot use non-qualified names with default_namespace option

But a default namespace declaration does not apply to attribute names (Namespaces in XML, 6.2), so an unqualified attribute name can be written as is. For the same reason a qualified attribute name always needs a prefix, even if it is in the default namespace, and its namespace has to be declared twice:

>>> e = ET.Element('{uri}svg', {'{uri}id': 'a'})
>>> ET.tostring(e, encoding='unicode', default_namespace='uri')
'<svg xmlns="uri" xmlns:ns1="uri" ns1:id="a" />'

lxml serializes it the same way. Attribute names are now encoded separately from element names, and the namespaces table maps prefixes to URIs, because one URI can need both the default declaration and a prefixed one.

This also fixes gh-113581, the same bug seen from the other side: a qualified attribute name in the default namespace was written without a prefix, so its namespace was silently lost when the document was parsed back.

• Issue: ElementTree incorrectly refuses to write attributes without namespaces when default_namespace is used #61290