gh-63102: Support custom targets in the pull parser - #156888

The list of events was collected by the TreeBuilder in the C implementation,
so the pull parser only worked with the standard target.  The parser itself
now collects the events, and reports what the target returns.  XMLPullParser
and iterparse() get the target parameter, which makes it possible to parse a
large document incrementally without building a tree for it.

The namespace events no longer need a separate code path: the parser reports
the prefix and the uri if the target does not implement start_ns()/end_ns(),
as the Python implementation already did.

TypeError is raised in the constructor if the target does not implement
start(), end(), comment() or pi() for the corresponding event, in both
implementations.  The C implementation silently reported nothing and
the Python implementation raised AttributeError while parsing.

…-target

# Conflicts:
#	Modules/_elementtree.c