Fix python bindings with versions older than 2.7

Need fixing on the Capsule usage, the lack of PyBytes,
lack of io module and the way to access exception details.

python/tests/xpathleak.py

@@ -42,7 +42,7 @@ badexprs = (
 for expr in badexprs:
 	try:
 		ctxt.xpathEval(expr)
-	except libxml2.xpathError as e:
+	except libxml2.xpathError:
 	        pass
 	else:
 		print("Unexpectedly legal expression:", expr)