display_method of objects with custom __getattr__

[Zaharid]

Some objects such as pymongo collections implement a custom __getattr__ method that always retursn an object.

Then for the IPython display there is https://github.com/ipython/ipython/blob/master/IPython/core/displayhook.py#L245 which will call the _ipython_display_ attribute returned by __getattr__ and produce unpredictable behaviour; best case just an error, like:

TypeError: 'Collection' object is not callable. If you meant to call the '_ipython_display_'
 method on a 'Collection' object it is failing because no such method exists.

Probably can be considered a bug of pymongo, but may be worth considering to check for existence in __dict__ rather than getattr.

[minrk]

See #5229.