Change how to determine the Python version

Change the way how to determine the Python version in [python38.ipynb](https://github.com/veit/jupyter-tutorial/blob/master/docs/workspace/jupyter/kernels/python38.ipynb) and [python39.ipynb](https://github.com/veit/jupyter-tutorial/blob/master/docs/workspace/jupyter/kernels/python39.ipynb).
```
In [1]: import sys
        !{sys.executable} --version
Python 3.8.0
```