Returns the list of possible drivers. Currently this starts out as
["xml.sax.drivers.drv_xmltok",
"xml.sax.drivers.drv_xmlproc",
"xml.sax.drivers.drv_xmltoolkit",
"xml.sax.drivers.drv_xmllib"].
Returns a SAX driver for the first available parser of the parsers
in the list. Note that the list contains drivers, so it first tries
the driver and if that exists imports it to see if the parser also
exists. If no parsers are available a SAXException is thrown.
Optionally, driver_name can be a string containing the name of
the driver to be used; the stored parser list will then not be used at
all.