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/common/test/test.py

@@ -133,7 +133,7 @@ class TestObjectID(unittest.TestCase):
         x = random_object_id()
         y = random_object_id()
         {x: y}
-        set([x, y])
+        {x, y}
 
 
 class TestTask(unittest.TestCase):