in_callback field that's set to true whenever a callback into an
    event handler is true.  Needed for:

set_error():  Add line number of offset information to the exception
    as attributes, so users don't need to parse the text of the
    message.

set_error_attr():  New helper function for set_error().

xmlparse_GetInputContext():  New function of the parser object;
    returns the document source for an event during a callback, None
    at all other times.

xmlparse_SetParamEntityParsing():  Make the signature consistent with
    the other parser methods (use xmlparseobject* for self instead of
    PyObject*).

initpyexpat():  Don't lose the reference to the exception class!

call_with_frame(),
getcode():  Re-indent to be consistent with the rest of the file.