restore tab completion behavior

this is a minimal patch to restore proper tab completion behavior, a
partial revert of  #8355. I think avoiding the explicit poking into the
__bases__ classes address the concerns  #8330.

IPython/utils/dir2.py

@@ -41,6 +41,9 b' def dir2(obj):'
         # TypeError: dir(obj) does not return a list
         words = set()
 
+    if safe_hasattr(obj, '__class__'):
+        words |= set(dir(obj.__class__))
+
     # filter out non-string attributes which may be stuffed by dir() calls
     # and poor coding in third-party modules