Fixed #17601 -- expose underlying db exceptions under py2

Use __cause__ to expose the underlying database exceptions even
under python 2.

django/db/utils.py

@@ -91,7 +91,6 @@ class DatabaseErrorWrapper(object):
                 except AttributeError:
                     args = (exc_value,)
                 dj_exc_value = dj_exc_type(*args)
-                if six.PY3:
                 dj_exc_value.__cause__ = exc_value
                 # Only set the 'errors_occurred' flag for errors that may make
                 # the connection unusable.

docs/ref/exceptions.txt

@@ -152,10 +152,16 @@ The Django wrappers for database exceptions behave exactly the same as
 the underlying database exceptions. See :pep:`249`, the Python Database API
 Specification v2.0, for further information.
 
+As per :pep:`3134`, a ``__cause__`` attribute is set with the original
+(underlying) database exception, allowing access to any additional
+information provided. (Note that this attribute is available under
+both Python 2 and Python 3, although :pep:`3134` normally only applies
+to Python 3.)
+
 .. versionchanged:: 1.6
 
     Previous version of Django only wrapped ``DatabaseError`` and
-    ``IntegrityError``.
+    ``IntegrityError``, and did not provide ``__cause__``.
 
 .. exception:: models.ProtectedError