test fails on Debian/Testing with traitlets 5.0.4 with python 3.8

[Trophime]

Hi, I'm trying to package traittypes for Debian.
I run into an error while running the test:

I: pybuild base:217: cd /build/traittypes-0.2.1/.pybuild/cpython3_3.8_traittypes/build; python3.8 -m pytest
============================= test session starts ==============================
platform linux -- Python 3.8.5, pytest-4.6.11, py-1.9.0, pluggy-0.13.0
rootdir: /build/traittypes-0.2.1
collected 3 items / 1 errors / 2 selected

==================================== ERRORS ====================================
_ ERROR collecting .pybuild/cpython3_3.8_traittypes/build/traittypes/tests/test_traittypes.py _
traittypes/tests/test_traittypes.py:9: in <module>
    from traitlets.tests.test_traitlets import TraitTestBase
<frozen importlib._bootstrap>:991: in _find_and_load
    ???
<frozen importlib._bootstrap>:975: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:655: in _load_unlocked
    ???
<frozen importlib._bootstrap>:618: in _load_backward_compatible
    ???
/usr/lib/python3/dist-packages/_pytest/assertion/rewrite.py:304: in load_module
    exec(co, mod.__dict__)
/usr/lib/python3/dist-packages/traitlets/tests/test_traitlets.py:1241: in <module>
    class UnionTraitTest(TraitTestBase):
/usr/lib/python3/dist-packages/traitlets/tests/test_traitlets.py:1243: in UnionTraitTest
    obj = UnionTrait(value='ipython_genutils.ipstruct.Struct')
/usr/lib/python3/dist-packages/traitlets/traitlets.py:1077: in __init__
    setattr(self, key, value)
/usr/lib/python3/dist-packages/traitlets/traitlets.py:604: in __set__
    self.set(obj, value)
/usr/lib/python3/dist-packages/traitlets/traitlets.py:578: in set
    new_value = self._validate(obj, value)
/usr/lib/python3/dist-packages/traitlets/traitlets.py:610: in _validate
    value = self.validate(obj, value)
/usr/lib/python3/dist-packages/traitlets/traitlets.py:1980: in validate
    self.error(obj, value)
/usr/lib/python3/dist-packages/traitlets/traitlets.py:690: in error
    raise TraitError(e)
E   traitlets.traitlets.TraitError: The 'value' trait of an UnionTrait instance expected a subclass of 'builtins.object' or a boolean, not the str 'ipython_genutils.ipstruct.Struct'.
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
=========================== 1 error in 0.36 seconds ============================
E: pybuild pybuild:352: test: plugin distutils failed with: exit code=2: cd /build/traittypes-0.2.1/.pybuild/cpython3_3.8_traittypes/build; python3.8 -m pytest
dh_auto_test: error: pybuild --test --test-pytest -i python{version} -p 3.8 returned exit code 13
make: *** [debian/rules:9: binary] Error 25

Now idea how to fix this.
Any help would be great.
Best