[XML-SIG] Better pyexpat backtraces

[Martin v. Loewis]

Since a number of people have run into the trap of thinking that Parse
is called with a bad argument number, I just checked-in a patch to
pyexpat that adds an artificial frame object on the stack. With that,
if you pass a DocumentHandler in place of a ContentHandler, you now
get a back-trace that reads

Traceback (most recent call last):
  File "a.py", line 48, in ?
    parser.parse( comic_xml )
  File "/usr/local/lib/python2.0/site-packages/_xmlplus/sax/expatreader.py", line 43, in parse
    xmlreader.IncrementalParser.parse(self, source)
  File "/usr/local/lib/python2.0/site-packages/_xmlplus/sax/xmlreader.py", line 120, in parse
    self.feed(buffer)
  File "/usr/local/lib/python2.0/site-packages/_xmlplus/sax/expatreader.py", line 87, in feed
    self._parser.Parse(data, isFinal)
  File "pyexpat.c", line 370, in CharacterData
TypeError: not enough arguments to characters(); expected 4, got 2

Normally, you would not get a stack frame that points to pyexpat.c;
please let me know what you think.

The "to characters()" part is not my doing; that is a Python 2.1
feature.

Regards,
Martin