Fix deprecation warning related to ndarrays
It seems it's only affecting a test but it's still annoying:
software/python/3.7.5/lib/python3.7/site-packages/km3pipe/tests/test_dataclasses.py::TestTable::test_append_columns_with_mismatching_lengths_raises
/pbs/throng/km3net/software/python/3.7.5/lib/python3.7/site-packages/numpy/core/_asarray.py:136: VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you must specify 'dtype=object' when creating the ndarray
return array(a, dtype, copy=False, order=order, subok=True)
-- Docs: https://docs.pytest.org/en/latest/warnings.html