Several changes:

1. Fix bug in (de)compression objects.  The final string resize used
zst.total_out to determine the length of the string, but the
(de)compression object will output data a little bit at a time, which
means total_out is not the string size.  Fix: save original value of
total_out at the start of the call.

2. Be sure to Py_DECREF the result value if you exit with an
exception.

3. Use PyInt_FromLong instead of Py_BuildValue

4. include more constants from the zlib header file

5. Use PyErr_Format instead of using a local buffer and sprintf.