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/experimental/tfutils.py

@@ -49,7 +49,7 @@ class TensorFlowVariables(object):
         self.sess = sess
         queue = deque([loss])
         variable_names = []
-        explored_inputs = set([loss])
+        explored_inputs = {loss}
 
         # We do a BFS on the dependency graph of the input function to find
         # the variables.