Implement __hash__ on Digest class

In Python 3, if a class defines __eq__() but not __hash__(), its
instances will not be usable as items in hashable collections (e.g sets).

digest/digest_tools.py

@@ -25,6 +25,9 @@ def __str__(self):
     def __eq__(self, rhs):
         return isinstance(rhs, Digest) and str(self) == str(rhs)
 
+    def __hash__(self):
+        return hash((self._hash_alg, self._hash_bytes))
+
     @staticmethod
     def parse_digest(digest):
         """ Returns the digest parsed out to its components. """