Trac #31523: Fix dependencies of ipywidgets

After #31278, which removed heavy dependencies from `widgetsnbextension`
(which is a dependency of `ipywidgets`), we have to add some (lighter)
dependencies to `ipywidgets`.

https://github.com/jupyter-widgets/ipywidgets/blob/master/setup.cfg#L35
lists:
{{{
install_requires =
  ipykernel>=4.5.1
  ipython>=6.1.0
  traitlets>=4.3.1
  nbformat>=4.2.0
  widgetsnbextension~=4.0a0
  jupyterlab_widgets~=2.0a0
}}}
These are not reflected in our `dependency` file. In this ticket we add
the first three. (`nbformat` is only there so as to rule out some broken
versions; we do not need `jupyterlab_...`.)

So far this has not caused a build failure, but it is possible that one
could appear in heavily parallelized builds.

The missing dependency is visible in the testsuite run for optional
package `p_group_cohomology` in
https://github.com/mkoeppe/sage/runs/2145415988
{{{
Running the test suite for p_group_cohomology-3.3.2...
Making check in src
Running doctests with ID 2021-03-19-18-58-47-d127e4fd.
Using --optional=debian,dochtml,meataxe,memlimit,pip,polylib,pycosat,sag
e,sage_spkg
Sorting sources by runtime so that slower doctests are run first....
Doctesting 13 files using 3 threads.
Traceback (most recent call last):
  File "/sage/src/bin/sage-runtests", line 182, in <module>
    err = DC.run()
  File "/sage/local/lib/python3.9/site-
packages/sage/doctest/control.py", line 1212, in run
    self.run_doctests()
  File "/sage/local/lib/python3.9/site-
packages/sage/doctest/control.py", line 909, in run_doctests
    self.dispatcher = DocTestDispatcher(self)
  File "/sage/local/lib/python3.9/site-packages/sage/doctest/forker.py",
line 1658, in __init__
    init_sage(controller)
  File "/sage/local/lib/python3.9/site-packages/sage/doctest/forker.py",
line 209, in init_sage
    controller.load_environment()
  File "/sage/local/lib/python3.9/site-
packages/sage/doctest/control.py", line 530, in load_environment
    return import_module(self.options.environment)
...
  File "/sage/local/lib/python3.9/site-
packages/sage/repl/ipython_kernel/all_jupyter.py", line 7, in <module>
    from .widgets_sagenb import (input_box, text_control, slider,
  File "/sage/local/lib/python3.9/site-
packages/sage/repl/ipython_kernel/widgets_sagenb.py", line 31, in
<module>
    from ipywidgets.widgets import (IntSlider, IntRangeSlider,
FloatSlider,
  File "/sage/local/lib/python3.9/site-packages/ipywidgets/__init__.py",
line 25, in <module>
    from .widgets import *
  File "/sage/local/lib/python3.9/site-
packages/ipywidgets/widgets/__init__.py", line 4, in <module>
    from .widget import Widget, CallbackDispatcher, register,
widget_serialization
  File "/sage/local/lib/python3.9/site-
packages/ipywidgets/widgets/widget.py", line 18, in <module>
    from ipykernel.comm import Comm
ModuleNotFoundError: No module named 'ipykernel'
************************************************************************
********
Error testing pGroupCohomology
}}}

URL: https://trac.sagemath.org/31523
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Jonathan Kliem

Author: Release Manager

build/pkgs/ipywidgets/dependencies

@@ -1,4 +1,4 @@
-$(PYTHON) widgetsnbextension | $(PYTHON_TOOLCHAIN)
+$(PYTHON) widgetsnbextension | $(PYTHON_TOOLCHAIN) ipykernel ipython traitlets
 
 ----------
 All lines of this file are ignored except the first.