Use flake8-comprehensions (#1976)

* Add flake8 to Travis

* Add flake8-comprehensions

[flake8 plugin](https://github.com/adamchainz/flake8-comprehensions) that
checks for useless constructions.

* Use generators instead of lists where appropriate

A lot of the builtins can take in generators instead of lists.

This commit applies `flake8-comprehensions` to find them.

* Fix lint error

* Fix some string formatting

The rest can be fixed in another PR

* Fix compound literals syntax

This should probably be merged after #1963.

* dict() -> {}

* Use dict literal syntax

dict(...) -> {...}

* Rewrite nested dicts

* Fix hanging indent

* Add missing import

* Add missing quote

* fmt

* Add missing whitespace

* rm duplicate pip install

This is already installed in another file.

* Fix indent

* move `merge_dicts` into utils

* Bring up to date with `master`

* Add automatic syntax upgrade

* rm pyupgrade

In case users want to still use it on their own, the upgrade-syn.sh script was
left in the `.travis` dir.

python/ray/monitor.py

@@ -97,7 +97,7 @@ class Monitor(object):
         self.dead_plasma_managers = set()
         # Keep a mapping from local scheduler client ID to IP address to use
         # for updating the load metrics.
-        self.local_scheduler_id_to_ip_map = dict()
+        self.local_scheduler_id_to_ip_map = {}
         self.load_metrics = LoadMetrics()
         if autoscaling_config:
             self.autoscaler = StandardAutoscaler(autoscaling_config,