Add jupyter_core as a dependency

We import it, but apparently forgot to require it.

setup.py

@@ -145,6 +145,7 @@ setup_args['install_requires'] = [
     'ipython',
     'packaging',
     'tornado!=6.0.0, !=6.0.1, !=6.0.2',
+    'jupyter_core',
     'jupyterlab_server~=2.0.0rc1',
     'jupyter_server~=1.0.1',
     'nbclassic~=0.2.0',