如何在此字典中仅获取中间嵌套键？

def get_nested_keys(dictionary):
    keys = []
    for key, value in dictionary.items():
        if isinstance(value, dict):
            nested_keys = get_nested_keys(value)
            keys.extend([key + '.' + nested_key for nested_key in nested_keys])
        else:
            keys.append(key)
    return keys

my_dict = {
    'key1': 'value1',
    'key2': {
        'nested_key1': 'nested_value1',
        'nested_key2': {
            'nested_nested_key1': 'nested_nested_value1',
            'nested_nested_key2': 'nested_nested_value2'
        },
        'nested_key3': 'nested_value3'
    },
    'key3': 'value3'
}

result = get_nested_keys(my_dict)
print(result)

['key2.nested_key2.nested_nested_key1', 'key2.nested_key2.nested_nested_key2']