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/test/test_utils.py

@@ -95,7 +95,7 @@ def _wait_for_event(event_name, redis_address, extra_buffer=0):
     redis_client = redis.StrictRedis(host=redis_host, port=int(redis_port))
     while True:
         event_infos = redis_client.lrange(EVENT_KEY, 0, -1)
-        events = dict()
+        events = {}
         for event_info in event_infos:
             name, data = json.loads(event_info)
             if name in events: