Disable too agressive flake8 errors

[asvetlov]

New flake8 tool blames too much.

I prefer to suppress innocent reported errors over fixing all reported lines.
Let's wait for black adoption to auto-reformat code without boring manual fixes.

[codecov-io]

Codecov Report

Merging #3425 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3425      +/-   ##
==========================================
- Coverage   97.92%   97.92%   -0.01%     
==========================================
  Files          44       44              
  Lines        8534     8626      +92     
  Branches     1378     1434      +56     
==========================================
+ Hits         8357     8447      +90     
  Misses         72       72              
- Partials      105      107       +2

Impacted Files	Coverage Δ
aiohttp/web_urldispatcher.py	99.25% <100%> (ø)	⬆️
aiohttp/base_protocol.py	97.46% <0%> (+0.54%)	⬆️
aiohttp/web_protocol.py	93.27% <0%> (+1.1%)	⬆️

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