pyexpat.errors is a *strange* module!

pyexpat.errors is a *strange* module!

It gets initialized when pyexpat is imported, and is only accessible as an
attribute of pyexpat; it cannot be imported itself.  This allows it to at
least be importable after pyexpat itself has been imported by adding it
to sys.modules, so it is not quite as strange.

This arrangement needs to be better thought out.