TypeError from traitlets when importing ipdb

Recently started getting this error when attempting to `import ipdb` in my tests:

```
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/ipdb/__init__.py:7: in
from ipdb.__main__ import set_trace, post_mortem, pm, run, runcall, runeval, launch_ipdb_on_exception
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/ipdb/__main__.py:47: in
ipapp.initialize([])
:2: in initialize
???
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/traitlets/config/application.py:87: in catch_config_error
return method(app, *args, **kwargs)
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/IPython/terminal/ipapp.py:317: in initialize
self.init_shell()
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/IPython/terminal/ipapp.py:333: in init_shell
ipython_dir=self.ipython_dir, user_ns=self.user_ns)
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/traitlets/config/configurable.py:412: in instance
inst = cls(*args, **kwargs)
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/IPython/terminal/interactiveshell.py:433: in __init__
super(TerminalInteractiveShell, self).__init__(*args, **kwargs)
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/IPython/core/interactiveshell.py:653: in __init__
self.init_history()
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/IPython/core/interactiveshell.py:1774: in init_history
self.history_manager = HistoryManager(shell=self, parent=self)
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/IPython/core/history.py:550: in __init__
self.save_thread = HistorySavingThread(self)
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/traitlets/traitlets.py:585: in __set__
self.set(obj, value)
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/traitlets/traitlets.py:559: in set
new_value = self._validate(obj, value)
/srv/venvs/service/trusty/service_venv_python3.6/lib/python3.6/site-packages/traitlets/traitlets.py:591: in _validate
value = self.validate(obj, value)
```

Python version 3.6
ipython==7.0.1
traitlets==4.3.2
ipdb==0.10.0

I am using the default IPython configuration.

I tried installing various combinations of versions for all these packages including the current `master` as well with no luck. Also tried modifying the `validate()` function in `traitlets` but that just caused other issues understandably.

Apologies if this isn't the correct project to be filing a bug with. Mostly looking for advice on how to debug this. Trying to follow the stacktrace is difficult and there is a lot of IPython specific initialization happening that I'd appreciate some guidance with.