Bump the ipywidgets version to the 7.0.0a1.

Parts of pythreejs now use the binary transfer mechanism in ipywidgets 7.

Author: jasongrout

setup.py

@@ -135,7 +135,7 @@ def run(self):
             'pythreejs/staticlab/jupyter-threejs.bundle.js.manifest',
         ]),
     ],
-    'install_requires': ['ipywidgets>=6.0.0', 'traittypes'],
+    'install_requires': ['ipywidgets>=7.0.0a1', 'traittypes'],
     'packages': find_packages(),
     'zip_safe': False,
     'cmdclass': {