Forbid inheritance from ClientSession and Application

[asvetlov]

No description provided.

[codecov-io]

Codecov Report

Merging #3933 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3933      +/-   ##
==========================================
+ Coverage   97.71%   97.72%   +0.01%     
==========================================
  Files          43       43              
  Lines        8795     8799       +4     
  Branches     1381     1383       +2     
==========================================
+ Hits         8594     8599       +5     
  Misses         86       86              
+ Partials      115      114       -1

Impacted Files	Coverage Δ
aiohttp/helpers.py	97.53% <100%> (+0.01%)	⬆️
aiohttp/web_app.py	99.17% <100%> (ø)	⬆️
aiohttp/http_parser.py	97.09% <0%> (+0.22%)	⬆️

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[kxepal]

Finally!