Add direct dependency on ipykernel

ipywidgets 8.1.0 has been released, which includes the change at
jupyter-widgets/ipywidgets#3811.
With this change, ipykernel is no longer a dependnecy of ipywidgets,
so jupyer-sphinx must now depend on ipykernel directly rather
than assume it will be installed as a transitive dependency of
ipywidgets.  Without this change, jupyter-sphinx tests will fail CI.
This has additionally led to CI failures in some packages that
depend on jupyter-sphinx but not ipykernel.

requirements.txt

@@ -1,4 +1,5 @@
 sphinx>=2.4.4
+ipykernel>=4.5.1
 ipywidgets>=7.0.0
 IPython
 nbconvert>=5.4