Use the ability to save the exception traceback with newer futures

This helps make things much nicer on py2.x release of python.

futurist/futures.py

@@ -15,6 +15,7 @@
 #    under the License.
 
 import functools
+import sys
 import threading
 
 from concurrent import futures as _futures
@@ -23,6 +24,7 @@ from concurrent.futures import thread as _thread
 from oslo_utils import importutils
 from oslo_utils import reflection
 from oslo_utils import timeutils
+import six
 
 greenpatcher = importutils.try_import('eventlet.patcher')
 greenpool = importutils.try_import('eventlet.greenpool')
@@ -174,8 +176,15 @@ class _WorkItem(object):
             return
         try:
             result = self.fn(*self.args, **self.kwargs)
-        except BaseException as e:
-            self.future.set_exception(e)
+        except BaseException:
+            exc_type, exc_value, exc_tb = sys.exc_info()
+            try:
+                if six.PY2:
+                    self.future.set_exception_info(exc_value, exc_tb)
+                else:
+                    self.future.set_exception(exc_value)
+            finally:
+                del(exc_type, exc_value, exc_tb)
         else:
             self.future.set_result(result)
 

setup.py

@@ -33,7 +33,7 @@ setup(
     license="ASL 2.0",
     install_requires=[
         # Only needed on 2.6 and 2.7 (can be killed/removed on 3.2+)
-        'futures',
+        'futures>=3.0',
         'six',
         'oslo.utils',
     ],