Call shutdown_asyncgens before loop closing

[asvetlov]

shutdown_asyncgens call is required on python 3.6

https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.AbstractEventLoop.shutdown_asyncgens

[codecov-io]

Codecov Report

Merging #2227 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2227      +/-   ##
==========================================
+ Coverage   97.15%   97.15%   +<.01%     
==========================================
  Files          39       39              
  Lines        7895     7899       +4     
  Branches     1367     1369       +2     
==========================================
+ Hits         7670     7674       +4     
  Misses        100      100              
  Partials      125      125

Impacted Files	Coverage Δ
aiohttp/web.py	99.66% <100%> (ø)	⬆️
aiohttp/worker.py	96.85% <100%> (+0.05%)	⬆️

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