bpo-35018: Sax parser should provide user access to lexical handlers (GH-20958)

Co-Authored-By: Jonathan Gossage <jgossage@gmail.com>

Lib/xml/sax/handler.py

@@ -340,3 +340,48 @@ all_properties = [property_lexical_handler,
                   property_xml_string,
                   property_encoding,
                   property_interning_dict]
+
+
+class LexicalHandler:
+    """Optional SAX2 handler for lexical events.
+
+    This handler is used to obtain lexical information about an XML
+    document, that is, information about how the document was encoded
+    (as opposed to what it contains, which is reported to the
+    ContentHandler), such as comments and CDATA marked section
+    boundaries.
+
+    To set the LexicalHandler of an XMLReader, use the setProperty
+    method with the property identifier
+    'http://xml.org/sax/properties/lexical-handler'."""
+
+    def comment(self, content):
+        """Reports a comment anywhere in the document (including the
+        DTD and outside the document element).
+
+        content is a string that holds the contents of the comment."""
+
+    def startDTD(self, name, public_id, system_id):
+        """Report the start of the DTD declarations, if the document
+        has an associated DTD.
+
+        A startEntity event will be reported before declaration events
+        from the external DTD subset are reported, and this can be
+        used to infer from which subset DTD declarations derive.
+
+        name is the name of the document element type, public_id the
+        public identifier of the DTD (or None if none were supplied)
+        and system_id the system identfier of the external subset (or
+        None if none were supplied)."""
+
+    def endDTD(self):
+        """Signals the end of DTD declarations."""
+
+    def startCDATA(self):
+        """Reports the beginning of a CDATA marked section.
+
+        The contents of the CDATA marked section will be reported
+        through the characters event."""
+
+    def endCDATA(self):
+        """Reports the end of a CDATA marked section."""