Incrementally parse XML document into ElementTree.
This class also reports what's going on to the user based on the
*events* it is initialized with. The supported events are the strings
"start", "end", "start-ns" and "end-ns" (the "ns" events are used to get
detailed namespace information). If *events* is omitted, only
"end" events are reported.
*source* is a filename or file object containing XML data, *events* is
a list of events to report back, *parser* is an optional parser
instance.
Returns an iterator providing (event, elem) pairs.