xml.dom.minidom writes a duplicate xmlns attribute #158208

Bug description:

Since gh-44376, writexml(), toxml() and toprettyxml() add xmlns="" to an element in no namespace even when the element has its own xmlns attribute, so the attribute is written twice and the output is not well-formed. This hits the usual way of building SVG or XHTML with createElement() and setAttribute("xmlns", ...), and documents parsed without namespace processing.

from xml.dom import minidom
doc = minidom.Document()
svg = doc.appendChild(doc.createElement("svg"))
svg.setAttribute("xmlns", "http://www.w3.org/2000/svg")
xml = doc.toxml()
print(xml)
try:
    minidom.parseString(xml)
except Exception as e:
    print(type(e).__name__, e)

<?xml version="1.0" ?><svg xmlns="" xmlns="http://www.w3.org/2000/svg"/>
ExpatError duplicate attribute: line 1, column 36

Expected: <?xml version="1.0" ?><svg xmlns="http://www.w3.org/2000/svg"/>, as on 3.14.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-158208: Don't write a duplicate xmlns attribute in xml.dom.minidom #158209
• gh-158208: Don't write a duplicate xmlns attribute in xml.dom.minidom #158211