Fix python3.x scoping issues with removed 'de' variable

In python3.x the 'de' variable will be removed from the
scope after the except block exits (if it ever is entered)
so we need to use a different variable name to ensure that
it will not be deleted so we can use it later.

This avoids errors of the format:

UnboundLocalError: local variable 'de' referenced before
                   assignment

Change-Id: I9ea2bd6b3a8c392f8d27b0130dd6b7683fc44e7c

oslo/db/sqlalchemy/session.py

@@ -569,7 +569,10 @@ def _test_connection(engine, max_retries, retry_interval):
         attempts = itertools.count()
     else:
         attempts = six.moves.range(max_retries)
-    de = None
+    # See: http://legacy.python.org/dev/peps/pep-3110/#semantic-changes for
+    # why we are not using 'de' directly (it can be removed from the local
+    # scope).
+    de_ref = None
     for attempt in attempts:
         try:
             return exc_filters.handle_connect_error(engine)
@@ -577,9 +580,10 @@ def _test_connection(engine, max_retries, retry_interval):
             msg = _LW('SQL connection failed. %s attempts left.')
             LOG.warning(msg, max_retries - attempt)
             time.sleep(retry_interval)
+            de_ref = de
     else:
-        if de is not None:
-            six.reraise(type(de), de)
+        if de_ref is not None:
+            six.reraise(type(de_ref), de_ref)
 
 
 class Query(sqlalchemy.orm.query.Query):