Since the return type of format() is not a Decimal, raise ValueError instead of
InvalidOperation if the format specification (width, prec) exceeds the internal
limits of libmpdec.
Issue #9708: Fix support for iterparse(parser=...) argument per documentation.

When _elementtree is imported, iterparse is redefined as a class and the parser
argument was ommitted. Fix this, and add a docstring to the class.
merge heads