PackWriter_Remote.close: call parent close

We weren't explicitly closing the PackWriter, and __del__ no longer
does either.  Make PackWriter a "new style" object so that the super()
call will also work with python 2.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: split from larger commit; adjust commit message]
[rlb@defaultvalue.org: convert PackWriter to "new style" object]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>