Bump bytecode version

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/compiler/Compiler.java

@@ -149,7 +149,7 @@
  * Compiler for bytecode interpreter.
  */
 public class Compiler implements SSTreeVisitor<Void> {
-    public static final int BYTECODE_VERSION = 20;
+    public static final int BYTECODE_VERSION = 21;
 
     ScopeEnvironment env;
     EnumSet<Flags> flags = EnumSet.noneOf(Flags.class);