Fix DeprecationWarnings (#1417)

* Fix DeprecationWarning with ipywidgets >= 7
* Fix MutableSequence DeprecationWarning with Python 3.7

Author: astrojuanlu

Diff for: plotly/basewidget.py

@@ -15,7 +15,7 @@
 from .serializers import custom_serializers
 from .version import __frontend_version__
 
-@widgets.register()
+@widgets.register
 class BaseFigureWidget(BaseFigure, widgets.DOMWidget):
     """
     Base class for FigureWidget. The FigureWidget class is code-generated as a

Diff for: plotly/grid_objs/grid_objs.py

@@ -5,7 +5,10 @@
 """
 from __future__ import absolute_import
 
-from collections import MutableSequence
+try:
+    from collections.abc import MutableSequence
+except ImportError:
+    from collections import MutableSequence
 
 from requests.compat import json as _json