Disable keepalives for websockets

[asvetlov]

Fix for #1955

[codecov-io]

Codecov Report

Merging #2701 into 2.3 will increase coverage by <.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##              2.3    #2701      +/-   ##
==========================================
+ Coverage   97.59%   97.59%   +<.01%     
==========================================
  Files          35       35              
  Lines        7432     7436       +4     
  Branches     1293     1294       +1     
==========================================
+ Hits         7253     7257       +4     
  Misses         71       71              
  Partials      108      108

Impacted Files	Coverage Δ
aiohttp/web_protocol.py	90.37% <83.33%> (+0.12%)	⬆️

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If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new