Clean up syntax for supported Python versions. (#1963)

* Use set/dict literal syntax

Ran code through [pyupgrade](https://github.com/asottile/pyupgrade). This is
supported in every Python version 2.7+.

* Drop unnecessary string format specification

No need to specify 0,1.. if paramters are passed in order.

* Revert "Drop unnecessary string format specification"

This reverts commit efa5ec85d30ff69f34e5ed93e31343fea7647bcb.

* Undo changes to cloudpickle

Drop use of set literal until cloudpickle uses it.

* Reformat code with YAPF

We need to set up a git pre-push hook to automatically run this stuff.

python/ray/worker.py

@@ -464,9 +464,11 @@ class Worker(object):
         final_results = self.retrieve_and_deserialize(plain_object_ids, 0)
         # Construct a dictionary mapping object IDs that we haven't gotten yet
         # to their original index in the object_ids argument.
-        unready_ids = dict((plain_object_ids[i].binary(), i)
-                           for (i, val) in enumerate(final_results)
-                           if val is plasma.ObjectNotAvailable)
+        unready_ids = {
+            plain_object_ids[i].binary(): i
+            for (i, val) in enumerate(final_results)
+            if val is plasma.ObjectNotAvailable
+        }
         was_blocked = (len(unready_ids) > 0)
         # Try reconstructing any objects we haven't gotten yet. Try to get them
         # until at least get_timeout_milliseconds milliseconds passes, then