I encountered this after upgrading ipython. An AttributeError in traitlets has stopped ipython from opening. I am not quite sure what's going on in the code after a brief peak, so thought I would bring it to your attention.

File "/opt/local/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/traitlets/config/loader.py", line 933, in __call__
    namespace._flags.append(self.flag)
AttributeError: 'Namespace' object has no attribute '_flags'

I am running OS X Mojave, python 3.9.8, and traitlets version 5.1.0. Please let me know if I can provide any more information or perform any tests that may be useful.