[Csv] csv to-do (fwd)

[Skip Montanaro]

I believe I took care of these items from the to-do list:

        - PyString_AsString()[0] on line 331 could return NULL in which case
          you are dereferencing a NULL pointer

        - PyString_AsString() can return NULL on line 1048 and 1063, the
          result is passed to join_append()

        - iteratable should be iterable?  (line 1088)

        - why doesn't csv_writerows() have a docstring?  csv_writerow does

        - any PyUnicode_* methods should be protected with #ifdef
          Py_USING_UNICODE 

        - csv_unregister_dialect, csv_get_dialect could use METH_O so you
          don't need to use PyArg_ParseTuple 

        - in init_csv, recommend using PyModule_AddIntConstant and
          PyModule_AddStringConstant where appropriate

Skip