plpython: fix memory leak

Attached is a patch that fixes three Python reference leaks in
PLy_traceback(): the objects returned by PyErr_Fetch() are owned by the
caller, so their reference count should be decremented. The memory leak
can be reproduced as follows:

create function import_fail() returns text as
'import foosocket
return "succeeded, that wasn''t supposed to happen"'
LANGUAGE plpythonu;

create function test_py() returns void as $$
begin
for i in 1 .. 1000 loop
perform import_fail();
end loop;
return;
end;$$ language plpgsql;

select test_py();

On my system, each invocation of test_py() leaks about 2MB. With the
patch applied, each invocation leaks about 500KB. So obviously there are
some more leaks here -- I searched briefly for additional problems, but
couldn't see anything obvious.

I don't have time at the moment to track down the remaining problems, so
I'd like to apply the patch as-is: I'll come back to the remaining
memory leaks later, unless someone beats me to it.

Barring any objections, I'll apply this patch to HEAD and back branches
tomorrow.

-Neil