Update britney-py.c to work with Python 2.5.

These changes are cherry-picked from britney1 (7e94274 and f50c039).
Commit messages follow.

7e94274: Use python 2.5.
f50c039: use PyObject_DEL() for memory allocated with PyObject_NEW().

See http://www.python.org/doc/2.5.4/whatsnew/ports.html (section about
"obmalloc"): Python 2.5 is no longer lenient about code that mixes NEW/DEL
from different memory families of functions.

lib/britney-py.c

@@ -1,4 +1,4 @@
-#include <python2.4/Python.h>
+#include <python2.5/Python.h>
 
 #include "dpkg.h"
 
@@ -54,7 +54,7 @@ static void dpkgpackages_dealloc(dpkgpackages *self) {
 	Py_XDECREF(self->ref);
 	self->pkgs = NULL;
 	self->ref = NULL;
-	PyMem_DEL(self);
+	PyObject_DEL(self);
 }
 
 
@@ -456,7 +456,7 @@ end:
 static void dpkgsources_dealloc(dpkgsources *self) {
 	free_sources(self->srcs);
 	self->srcs = NULL;
-	PyMem_DEL(self);
+	PyObject_DEL(self);
 }
 
 static PyObject *dpkgsources_packages(dpkgsources *self, PyObject *args)
@@ -678,7 +678,7 @@ static void dpkgsrcsn_dealloc(dpkgsrcsn *self) {
 	Py_XDECREF(self->refs);
 	self->refs = NULL;
 
-	PyMem_DEL(self);
+	PyObject_DEL(self);
 }
 
 static PyObject *dpkgsrcsn_removesource(dpkgsrcsn *self, PyObject *args) {