Cleaner handling of errors at interpreter shutdown on Py2. Ref #802.

src/gevent/_compat.py

@@ -29,7 +29,7 @@ else:
 ## Exceptions
 if PY3:
     def reraise(t, value, tb=None): # pylint:disable=unused-argument
-        if value.__traceback__ is not tb:
+        if value.__traceback__ is not tb and tb is not None:
             raise value.with_traceback(tb)
         raise value
 

src/gevent/_tblib.py

@@ -329,6 +329,10 @@ def install():
 
 def _unlocked_imports(f):
     def g(a):
+        if sys is None: # pragma: no cover
+            # interpreter shutdown on Py2
+            return
+
         gb = None
         if 'gevent.builtins' in sys.modules:
             gb = sys.modules['gevent.builtins']