[3.7] Fix python 3.8 warnings (#4264).

[asvetlov]

(cherry picked from commit 159b039)

Co-authored-by: Andrew Svetlov andrew.svetlov@gmail.com

[codecov-io]

Codecov Report

Merging #4265 into 3.7 will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              3.7    #4265   +/-   ##
=======================================
  Coverage   97.96%   97.96%           
=======================================
  Files          44       44           
  Lines        8886     8886           
  Branches     1392     1392           
=======================================
  Hits         8705     8705           
  Misses         72       72           
  Partials      109      109

Impacted Files	Coverage Δ
aiohttp/connector.py	97.43% <ø> (ø)	⬆️
aiohttp/web_server.py	100% <100%> (ø)	⬆️
aiohttp/locks.py	92.85% <100%> (ø)	⬆️

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