test(fix): make tests run with nbconvert >=6,<7

[maartenbreddels]

Closes #5086
Related #5085
jupyter/nbconvert#1045

[maartenbreddels]

PS: It seems that only the tests were affected.

[takluyver]

Adding a specific 'except for nbconvert' workaround doesn't seem like the right fix for the long term.

Maybe rather than patching out the entire of jupyter_path() to temporary directories, we should be patching out the user and system directories, and leaving ENV_JUPYTER_PATH alone, so it can access files installed along with the importable Python packages.

That would mean that the tests can still be isolated by using an environment system, but they don't try to isolate themselves from data files in the environment.

[maartenbreddels]

@takluyver I agree, that makes more sense.

[takluyver]

#5111 is my attempt at that.

[takluyver]

Thanks - closing this one as I've merged #5111.