Looks like this would break backward compatibility. Are there any advantages besides the improved code completion?

Better type hints than just integer was really what I was trying to achieved here. Of course that can be achieved with a literal type (Literal[1, 2]), but a enum makes a better connection between the sentinel value and their function, and gives better auto code completion, as you said.
I went specifically with enum.IntEnum in an attempt to not brake anything.XMLParser.DOM==1 andXMLParser.SAX==2 will evaluate to true, so you can still pass a literal 1 or 2 too the methods, as opposed to a enum.Enum class.
I can of course reintroduce the constants XML_PARSER_DOM and XML_PARSER_SAX as both as 1 and 2 or as XMLParser.DOM and XMLParser.SAX respectably.