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New version:
 - if __build_class__ is missing, raise a NameError instead of surprising ImportError
 - add tests
 - if PyObject_GetItem on __builtins__ or globals fail, only raise NameError if the exception is a KeyError
Before my patch, a new dict was created for builtins if __builtins__ exists in global but is not a dict. With my patch, the __builtins__ is kept and the type is checked at runtime. If __builtins__ is not a mapping, an exception is raised on lookup in ceval.c.
We may check __builtins__ type in PyFrame_New() using:
PyDict_Check(builtins) || (PyMapping_Check(mapping) && !PyList_Check(mapping) && !PyTuple_Check(mapping))
(PyDict_Check(builtins) is checked first for performance)