Some Error while updating Python in conda

```
from notebook.nbextensions import main
File "/home/shreyas/Softwares/anaconda3/lib/python3.6/site-packages/notebook/__init__.py", line 25, in
from .nbextensions import install_nbextension
File "/home/shreyas/Softwares/anaconda3/lib/python3.6/site-packages/notebook/nbextensions.py", line 31, in
from .config_manager import BaseJSONConfigManager
File "/home/shreyas/Softwares/anaconda3/lib/python3.6/site-packages/notebook/config_manager.py", line 14, in
from six import PY3
ModuleNotFoundError: No module named 'six'

WARNING conda.core.link:run_script(1099): pre-unlink script failed for package conda-forge::widgetsnbextension-3.4.2-py36_1000
consider notifying the package maintainer

```

I got this while updating Python to 3.7.2 in conda! Please note.

Start by reproducing the Python 3.7.2 update in conda and review the traceback through notebook/__init__.py, nbextensions.py, and config_manager.py, along with the widgetsnbextension pre-unlink warning. Determine why six is unavailable during the update; done means the update completes without the ModuleNotFoundError or pre-unlink failure.

Written by the indexing model from the issue text.