This is the last version actually supported by ipykernel.
* gnu/packages/python-xyz.scm (python2-ipython): Downgrade to 4.10.1.
[arguments]: Remove custom 'check phase.
[propagated-inputs]: Add python2-tornado, python2-traitlets.
[native-inputs]: Add python2-mock, python2-pytest-cov.
(python-ipykernel)[properties): Declare python2-ipykernel as the
python2- variant.