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.

examples/resnet/resnet_main.py

@@ -230,7 +230,7 @@ def train():
                 # testing task with the current weights every 200 steps.
                 acc = ray.get(acc_id)
                 acc_id = test_actor.accuracy.remote(weight_id, step)
-                print("Step {0}: {1:.6f}".format(step - 200, acc))
+                print("Step {}: {:.6f}".format(step - 200, acc))
     except KeyboardInterrupt:
         pass