Notebook sets PYTHONPATH via sys.path.append

[fredm73]

I am reporting a bug. At the start of this experiment, my Windows PYTHONPATH environment variable is not set.
When, in Spyder, I execute '_sys.path.append("D:\Documents\Courses\fastaiMachineLearning\ml1\fastai")' the PYTHONPATH remains unset (as it should). Things work fine in my Spyder session and subsequent ones.

When in Jupyter notebook, I execute the same 'sys.path' statement, my PYTHONPATH is set (by Jupyter, I guess). This is unfortunate, because I have a module
'IO.py' in the fastai folder. Now, when I execute the Anaconda prompt I get

Fatal Python error: init_sys_streams: can't initialize sys standard streams
Traceback (most recent call last):
  File "D:\Documents\Courses\fastaiMachineLearning\ml1\fastai\io.py", line 1, in <module>
ImportError: attempted relative import with no known parent package

I get the same error now when starting up anything else that uses Python (like Spyder).

Now, when I remove the environment variable PYTHONPATH from Windows things work fine again.
I believe it is a bug for the PYTHONPATH to be set (by Juypter notebook via the sys.path.append). How can I set the sys.path temporarily in the notebook without the environment variable being set?

[takluyver]

Neither Jupyter nor IPython sets PYTHONPATH as far as I know.