core: Implement Dead.__ne__ & Dead.__hash__

Both these addtions are to address warnings in
https://lgtm.com/projects/g/dw/mitogen/alerts/?mode=list. Namely that if
a class defines an equality method then it should also define an
inequality and a hash method.

Refs #61

mitogen/core.py

@@ -121,9 +121,15 @@ class TimeoutError(Error):
 
 
 class Dead(object):
+    def __hash__(self):
+        return hash(Dead)
+
     def __eq__(self, other):
         return type(other) is Dead
 
+    def __ne__(self, other):
+        return type(other) is not Dead
+
     def __reduce__(self):
         return (_unpickle_dead, ())