This (old) patch needs some work:

- unit tests are neeeded.

- it it not enough to return NULL when XML_ParserReset() returns an
error; a python exception must be raised.

- "This function may not be used on a parser created using
XML_ExternalEntityParserCreate": if XML_ParserReset() does not check it
properly, we should ensure that an exception is properly raised.

- "How many of the struct elements do I need to reset?": this should be
worked out. And I'm not convinced that self->intern should be reset.