Refactor ClientResponse constructor

[asvetlov]

1. Make logically required constructor arguments mandatory.
2. Drop _post_init() method

[codecov-io]

Codecov Report

Merging #2820 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2820      +/-   ##
==========================================
- Coverage   97.99%   97.97%   -0.02%     
==========================================
  Files          39       39              
  Lines        7385     7375      -10     
  Branches     1297     1295       -2     
==========================================
- Hits         7237     7226      -11     
  Misses         47       47              
- Partials      101      102       +1

Impacted Files	Coverage Δ
aiohttp/client_reqrep.py	97.24% <100%> (-0.23%)	⬇️

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[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
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