[3.7] Use loop.sendfile() instead of custom implementation if available (#4517)

[asvetlov]

(cherry picked from commit e6f04ce)

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

[codecov-io]

Codecov Report

Merging #4518 into 3.7 will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              3.7    #4518      +/-   ##
==========================================
- Coverage   97.93%   97.88%   -0.05%     
==========================================
  Files          44       44              
  Lines        8945     8953       +8     
  Branches     1406     1407       +1     
==========================================
+ Hits         8760     8764       +4     
- Misses         78       81       +3     
- Partials      107      108       +1

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	97.82% <100%> (-0.47%)	⬇️
aiohttp/pytest_plugin.py	95.75% <0%> (-1.82%)	⬇️

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