stdlib/xml/sax: Add type annotations (#10606)

* stdlib/xml/sax: Type annotations for commonly used methods.

* stdlib/xml/sax: More annotations.

It turns out SAX's definition of a "qname" is exactly the opposite of
ElementTree's. With that understanding, let's annotate the Attributes*Impl
classes too.

* stdlib/xml/sax: I better understand what AttributesNSImpl is doing now.

* Update third-party library stubs to agree with the new SAX annotations.

stubs/untangle/untangle.pyi

@@ -1,7 +1,7 @@
 from collections.abc import Iterator, Mapping
 from typing import Any
 from typing_extensions import Self
-from xml.sax import handler
+from xml.sax import handler, xmlreader
 
 def is_string(x: object) -> bool: ...
 
@@ -29,7 +29,7 @@ class Handler(handler.ContentHandler):
     root: Element
     elements: list[Element]
     def __init__(self) -> None: ...
-    def startElement(self, name: str, attributes: Mapping[str, Any]) -> None: ...
+    def startElement(self, name: str, attributes: xmlreader.AttributesImpl) -> None: ...
     def endElement(self, name: str) -> None: ...
     def characters(self, cdata: str) -> None: ...