Fix exception when unpickling operator objects

Discovered when debugging a deadlock when evaluating a roll in a process pool
executor. The use of the negate (-) operator was enough to trigger the bug.

Pickle is used to de/serialize data when communicating with worker processes,
but it was unable to reconstruct the operator object because of missing
arguments to `__new__`. Overriding the `__getnewargs__` method to return
the necessary arguments solved the problem, since the default is assumed to
be an empty tuple.

reprex:
```py
r = dice.roll('-d20', raw=True)
d = pickle.dumps(r)
pickle.loads(d)
```

Author: calebj

dice/elements.py

@@ -393,6 +393,9 @@ def __repr__(self):
             classname(self), ", ".join(map(str, self.original_operands))
         )
 
+    def __getnewargs__(self):
+        return self.original_operands
+
     def preprocess_operands(self, *operands, **kwargs):
         def eval_wrapper(operand):
             return self.evaluate_object(operand, **kwargs)