gh-101100: Fix sphinx warnings in library/xml.sax.handler.rst

[LamentXU123]

There are two warnings:

C:\Users\admin\Downloads\cpython-main\Doc\library\xml.sax.handler.rst:249: WARNING: py:class reference target not found: xml.sax.xmlreader.Attributes [ref.class]
C:\Users\admin\Downloads\cpython-main\Doc\library\xml.sax.handler.rst:271: WARNING: py:class reference target not found: xml.sax.xmlreader.AttributesNS [ref.class]

This warning happens in many doc of xml (xml.sax.reader, xml.sax.handler, xml.sax) in the same reason. After staring the doc for a while, I think the original doc want to refer to xml.sax.xmlreader.AttributesImpl and xml.sax.xmlreader.AttributesNSImpl as an implemention of attributes-interface. This doc fix it to the correct ref.

If this is approved, I will fix up those occurrence in other docs, an example of this in xml.sax is as below to double check.

• Issue: Fix all Sphinx reference warnings in the documentation #101100

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📚 Documentation preview 📚: https://cpython-previews--136612.org.readthedocs.build/

[LamentXU123]

Fixed to: