Add error checking to __prepare__ call

Author: msimacek

graalpython/com.oracle.graal.python.test/src/tests/unittest_tags/test_types.txt

@@ -1,3 +1,4 @@
+*graalpython.lib-python.3.test.test_types.ClassCreationTests.test_bad___prepare__
 *graalpython.lib-python.3.test.test_types.ClassCreationTests.test_metaclass_derivation
 *graalpython.lib-python.3.test.test_types.ClassCreationTests.test_metaclass_override_callable
 *graalpython.lib-python.3.test.test_types.ClassCreationTests.test_metaclass_override_function

graalpython/lib-graalpython/classes.py

@@ -103,6 +103,9 @@ def prepare_class(name, bases=(), kwds=None):
         meta = _calculate_meta(meta, bases)
     if hasattr(meta, '__prepare__'):
         ns = meta.__prepare__(name, bases, **kwds)
+        if not hasattr(ns, '__getitem__'):
+            raise TypeError("%s.__prepare__() must return a mapping, not %s" %
+                            (meta.__name__ if isinstance(meta, type) else '<metaclass>', type(ns).__name__))
     else:
         ns = {}
     return meta, ns, kwds