如何在Python中仅复制嵌套字典的键结构？

def copy_dict_structure(dictionary):
    if isinstance(dictionary, dict):
        new_dict = {}
        for key, value in dictionary.items():
            new_dict[key] = copy_dict_structure(value)
        return new_dict
    else:
        return None

original_dict = {
    'key1': {
        'key2': {
            'key3': 'value3',
            'key4': 'value4'
        },
        'key5': 'value5'
    },
    'key6': 'value6'
}

new_dict = copy_dict_structure(original_dict)
print(new_dict)

{
    'key1': {
        'key2': {
            'key3': None,
            'key4': None
        },
        'key5': None
    },
    'key6': None
}