* Remove all "loop=self.loop" expressions.

* Rely on the currently running loop in the constructor of Queue.

* Assuming that janus.Queue objects are created in the functions or
  coroutines called by the event loop, rewrite most test cases to
  be async.

  - No longer manage the event loop lifecycles by ourselves.

  - Adopt pytest-asyncio to seamlessly run test cases in an event loop.

* Add missing .close() / .wait_closed() calls to the end of many test
  cases to ensure proper termination of the queues.

* Insert asyncio.sleep(0) in the wait_closed() method so that all
  task-done callbacks for tasks spawned by _notify_async_not_empty(),
  _notify_async_not_full() internal methods are properly awaited.
  This eliminates hundreds of resource warnings after finishing the test
  suite.

* Ensure dropping of Python 3.3/3.4 in CI configs.

* Add Python 3.7 and 3.8 to CI configs.

* Remove PY_33/PY_35 conditional branches as we no longer support Python
  3.4 or older versions.