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/tune/test/trial_scheduler_test.py

@@ -688,36 +688,36 @@ class PopulationBasedTestingSuite(unittest.TestCase):
         # Categorical case
         assertProduces(
             lambda: explore({"v": 4}, {"v": [3, 4, 8, 10]}, 0.0, lambda x: x),
-            set([3, 8]))
+            {3, 8})
         assertProduces(
             lambda: explore({"v": 3}, {"v": [3, 4, 8, 10]}, 0.0, lambda x: x),
-            set([3, 4]))
+            {3, 4})
         assertProduces(
             lambda: explore({"v": 10}, {"v": [3, 4, 8, 10]}, 0.0, lambda x: x),
-            set([8, 10]))
+            {8, 10})
         assertProduces(
             lambda: explore({"v": 7}, {"v": [3, 4, 8, 10]}, 0.0, lambda x: x),
-            set([3, 4, 8, 10]))
+            {3, 4, 8, 10})
         assertProduces(
             lambda: explore({"v": 4}, {"v": [3, 4, 8, 10]}, 1.0, lambda x: x),
-            set([3, 4, 8, 10]))
+            {3, 4, 8, 10})
 
         # Continuous case
         assertProduces(
             lambda: explore(
                 {"v": 100}, {"v": lambda: random.choice([10, 100])}, 0.0,
                 lambda x: x),
-            set([80, 120]))
+            {80, 120})
         assertProduces(
             lambda: explore(
                 {"v": 100.0}, {"v": lambda: random.choice([10, 100])}, 0.0,
                 lambda x: x),
-            set([80.0, 120.0]))
+            {80.0, 120.0})
         assertProduces(
             lambda: explore(
                 {"v": 100.0}, {"v": lambda: random.choice([10, 100])}, 1.0,
                 lambda x: x),
-            set([10.0, 100.0]))
+            {10.0, 100.0})
 
     def testYieldsTimeToOtherTrials(self):
         pbt, runner = self.basicSetup()

python/ray/tune/trial_runner.py

@@ -172,7 +172,7 @@ class TrialRunner(object):
             if max_debug == start_num:
                 break
 
-        for local_dir in sorted(set([t.local_dir for t in self._trials])):
+        for local_dir in sorted({t.local_dir for t in self._trials}):
             messages.append("Result logdir: {}".format(local_dir))
         for state, trials in sorted(states.items()):
             limit = limit_per_state[state]