Add deprecated description_tooltip for backwards compatibility.

This follows up on #2680 to help users transition to this backwards-incompatible change.

docs/source/user_migration_guides.md

@@ -30,12 +30,10 @@ to wrap reads from the widget.
 
 As part of an effort to make it possible to
 [set tooltips for all widgets](https://github.com/jupyter-widgets/ipywidgets/pull/2680),
-the old `description_tooltip` attribute for certain widgets was removed. Now all widgets
+the old `description_tooltip` attribute for certain widgets was deprecated. Now all widgets
 that inherit `DOMWidget` have the attribute `tooltip` instead.
 
 Suggested migration: Search and replace `description_tooltip` to `tooltip`.
-TBD: ipywidgets should add a `description_tooltip` keyword argument to `DescriptionWidget`s, with
-a deprecation warning.
 
 #### Selection Widgets
 

python/ipywidgets/ipywidgets/widgets/widget_description.py

@@ -9,6 +9,7 @@
 from .widget_style import Style
 from .widget_core import CoreWidget
 from .domwidget import DOMWidget
+import warnings
 
 @register
 class DescriptionStyle(Style, CoreWidget, Widget):
@@ -24,6 +25,13 @@ class DescriptionWidget(DOMWidget, CoreWidget):
     description_allow_html = Bool(False, help="Accept HTML in the description.").tag(sync=True)
     style = InstanceDict(DescriptionStyle, help="Styling customizations").tag(sync=True, **widget_serialization)
 
+    def __init__(self, *args, **kwargs):
+        if 'description_tooltip' in kwargs:
+            warnings.warn("the description_tooltip argument is deprecated, use tooltip instead", DeprecationWarning)
+            kwargs.setdefault('tooltip', kwargs['description_tooltip'])
+            del kwargs['description_tooltip']
+        super().__init__(*args, **kwargs)
+
     def _repr_keys(self):
         for key in super()._repr_keys():
             # Exclude style if it had the default value
@@ -32,3 +40,17 @@ def _repr_keys(self):
                 if repr(value) == '%s()' % value.__class__.__name__:
                     continue
             yield key
+
+    @property
+    def description_tooltip(self):
+        """The tooltip information.
+        .. deprecated :: 8.0.0
+           Use tooltip attribute instead.
+        """
+        warnings.warn(".description_tooltip is deprecated, use .tooltip instead", DeprecationWarning)
+        return self.tooltip
+
+    @description_tooltip.setter
+    def description_tooltip(self, tooltip):
+        warnings.warn(".description_tooltip is deprecated, use .tooltip instead", DeprecationWarning)
+        self.tooltip = tooltip