Notifying a trait with a DataFrame instance throws Value Error

Hey there,

At first, thank you for this amazing library!

I noticed that there are problems when linking multiple objects using the link function, if the value of a trait is a dataframe.
To me it looks like the compare logic overridden by pandas is causing the problem.
Here is an example:

```python
import traitlets
from traitlets import link, directional_link
import pandas as pd

class SomeClass(traitlets.HasTraits):
df = traitlets.Instance(klass=pd.DataFrame, allow_none=True)

foo = SomeClass()
baz = SomeClass()
bar = SomeClass()

# Will not work
link((foo, "df"), (baz, "df"))
foo.df = pd.DataFrame() # Throws ValueError
```

Stacktrace:

```python
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
traitlets_dataframe.ipynb Cell 4' in ()
2 link((foo, "df"), (baz, "df"))
4 # Throws ValueError
----> 5 foo.df = pd.DataFrame()

File .\lib\site-packages\traitlets\traitlets.py:712, in TraitType.__set__(self, obj, value)
710 raise TraitError('The "%s" trait is read-only.' % self.name)
711 else:
--> 712 self.set(obj, value)

File .\lib\site-packages\traitlets\traitlets.py:701, in TraitType.set(self, obj, value)
697 silent = False
698 if silent is not True:
699 # we explicitly compare silent to True just in case the equality
700 # comparison above returns something other than True/False
--> 701 obj._notify_trait(self.name, old_value, new_value)

File .\lib\site-packages\traitlets\traitlets.py:1371, in HasTraits._notify_trait(self, name, old_value, new_value)
1370 def _notify_trait(self, name, old_value, new_value):
-> 1371 self.notify_change(
1372 Bunch(
1373 name=name,
1374 old=old_value,
1375 new=new_value,
1376 owner=self,
1377 type="change",
1378 )
1379 )

File .\lib\site-packages\traitlets\traitlets.py:1383, in HasTraits.notify_change(self, change)
1381 def notify_change(self, change):
1382 """Notify observers of a change event"""
-> 1383 return self._notify_observers(change)

File .\lib\site-packages\traitlets\traitlets.py:1428, in HasTraits._notify_observers(self, event)
1425 elif isinstance(c, EventHandler) and c.name is not None:
1426 c = getattr(self, c.name)
-> 1428 c(event)

File .\lib\site-packages\traitlets\traitlets.py:366, in link._update_target(self, change)
364 with self._busy_updating():
365 setattr(self.target[0], self.target[1], self._transform(change.new))
--> 366 if getattr(self.source[0], self.source[1]) != change.new:
367 raise TraitError(
368 "Broken link {}: the source value changed while updating "
369 "the target.".format(self)
370 )

File .\lib\site-packages\pandas\core\generic.py:1527, in NDFrame.__nonzero__(self)
1525 @final
1526 def __nonzero__(self):
-> 1527 raise ValueError(
1528 f"The truth value of a {type(self).__name__} is ambiguous. "
1529 "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
1530 )

ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
```

As you can see, hhe problem is in `traitlets.py: 366`, because `getattr(self.source[0], self.source[1]) != change.new` does not return a bool value in the case of a DataFrame.

Would it be possible to make this function compatible with pandas, or possibly define a custom function for comparison?

Thank you in advance!