POM: update Jython version to 2.5.3

This is very important because 2.5.2 is not actually available from
Maven Central. We never noticed that this project will not build on a
clean machine, because we only ever built it on machines that already
had Jython 2.5.2 available from the Maven local repository cache...

Author: ctrueden

pom.xml

@@ -91,7 +91,7 @@
 		<dependency>
 			<groupId>org.python</groupId>
 			<artifactId>jython</artifactId>
-			<version>2.5.2</version>
+			<version>2.5.3</version>
 		</dependency>
 
 		<!-- Test dependencies -->