Fix up Python 3.8 loop argument warnings

[achimnol]

What do these changes do?

• 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 and loop termination after using .join() APIs.
• Ensure dropping of Python 3.3/3.4 in CI configs.
• Add Python 3.7 and 3.8 to CI configs.

Are there changes in behavior for the user?

Users are no longer able to create janus.Queue instances in the codes running outside event loops (even when they have references to an existing event loop running somewhere else).

Related issue number

#229

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • There is no CHANGES folder, so I've added a bullet in CHANGES.rst directly.

[codecov]

Codecov Report

Merging #246 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #246   +/-   ##
=======================================
  Coverage   99.67%   99.67%           
=======================================
  Files           1        1           
  Lines         308      309    +1     
  Branches       40       41    +1     
=======================================
+ Hits          307      308    +1     
  Partials        1        1           

Impacted Files	Coverage Δ
janus/__init__.py	99.67% <100.00%> (+<0.01%)	⬆️

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[achimnol]

@asvetlov Just a poke, please have a look at this PR.

[asvetlov]

Awesome!
Very impressive work, thanks!