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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def resolve_class(classref):
 """Attempt to return a Python class for the input class reference. If `classref` is a class or None, return it. If `classref` is a python classpath (e.g., "foo.bar.MyClass") import the class and return it. Args: classref: A fully-qualified Python path to class, or a Python class. Returns: A class. """

    if classref is None:
        return None
    elif isinstance(classref, six.class_types):
        return classref
    elif isinstance(classref, six.string_types):
        return import_class(classref)
    else:
        raise ValueError("Unable to resolve class for '%s'" % classref)