no display name and no $DISPLAY environment variable in Jupyter Notebook

I want to teach Python for kids. I want them to use Jupyter Notebook. To avoid the hassle of installation, I want them to use a free online Jupyter Notebook. I thought google's colab ([link](https://colab.research.google.com/notebooks) and jupyter.org ([link](https://jupyter.org/try) could be best solutions. However, I tried them both and got the error message below:
```
from turtle import *
colors = ['red', 'purple', 'blue', 'green', 'yellow', 'orange']
for x in range(360):
pencolor(colors[x % 6])
width(x / 100 + 1)
forward(x)
left(59)
```

TclError: no display name and no $DISPLAY environment variable

该 issue 没有指出任何 repository 文件或测试；首先在 Google Colab 和报告中链接的 Jupyter 试用环境中复现 turtle 示例。确定教程是否需要针对特定环境的指导，并在明确记录受支持的在线设置和预期行为后，视为该 issue 已完成。

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