pgsql: Fix three Python reference leaks in PLy_traceback().

Log Message:
-----------
Fix three Python reference leaks in PLy_traceback(). This would result
in leaking memory when invoking a PL/Python procedure that raises an
exception. Unfortunately this still leaks memory, but at least the
largest leak has been plugged.

This patch also fixes a reference counting mistake in PLy_modify_tuple()
for 8.0, 8.1 and HEAD: we don't actually own a reference to `platt', so
we shouldn't Py_DECREF() it.

Modified Files:
--------------
    pgsql/src/pl/plpython:
        plpython.c (r1.70 -> r1.71)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c.diff?r1=1.70&r2=1.71)