Add constant 'sys.maxunicode'.

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java

@@ -174,6 +174,7 @@ public void initialize(PythonCore core) {
                         2,                      // FLT_RADIX
                         1                       // FLT_ROUNDS
         }));
+        builtinConstants.put("maxunicode", Character.MAX_CODE_POINT);
 
         String os = getPythonOSName();
         builtinConstants.put("platform", os);