Merge pull request #518 from anikamukherji/immutable-causes

use object.setattr to bypass check for immutable objects

Author: jmadler

src/future/utils/__init__.py

@@ -475,7 +475,7 @@ def raise_from(exc, cause):
             e.__suppress_context__ = True
         elif isinstance(cause, BaseException):
             e.__cause__ = cause
-            e.__cause__.__traceback__ = sys.exc_info()[2]
+            object.__setattr__(e.__cause__,  '__traceback__', sys.exc_info()[2])
             e.__suppress_context__ = True
         else:
             raise TypeError("exception causes must derive from BaseException")