JupyterLab 3 only works on python >=3.5, so only depend on jupyterlab_widgets if python >=3.5.

Fixes #3049

Author: jasongrout

setup.py

@@ -119,13 +119,13 @@
     # only if notebook 4.x is installed in this
     # interpreter, to allow ipywidgets to be
     # installed on bare kernels.
-    'widgetsnbextension~=3.5.0',
-    'jupyterlab_widgets>=1.0.0',
+    'widgetsnbextension~=3.5.0'
 ]
 
 extras_require = setuptools_args['extras_require'] = {
     ':python_version<"3.3"' : ['ipython>=4.0.0,<6.0.0'],
     ':python_version>="3.3"': ['ipython>=4.0.0'],
+    ':python_version>="3.5"': ['jupyterlab_widgets>=1.0.0'],
     'test:python_version=="2.7"': ['mock'],
     'test': ['pytest>=3.6.0', 'pytest-cov'],
 }