Ugly fix used when pyexpat is not available.

If pyexpat is not available and more than one attempt is made to load
an expat-based xml parser, an empty xml.parser.expat module will be
created.  This empty module will confuse xml.sax.expatreader into
thinking that pyexpat is available.

The ugly fix is to verify that the expat module actually defines the
names that are imported from pyexpat.