Backport contextvars support

[asvetlov]

The code is on the master already

[codecov-io]

Codecov Report

Merging #4271 into 3.7 will decrease coverage by 0.05%.
The diff coverage is 91.48%.

@@            Coverage Diff             @@
##              3.7    #4271      +/-   ##
==========================================
- Coverage   97.96%   97.91%   -0.06%     
==========================================
  Files          44       44              
  Lines        8886     8905      +19     
  Branches     1392     1391       -1     
==========================================
+ Hits         8705     8719      +14     
- Misses         72       76       +4     
- Partials      109      110       +1

Impacted Files	Coverage Δ
aiohttp/http_parser.py	97.71% <100%> (+0.01%)	⬆️
aiohttp/web_protocol.py	90.88% <91.11%> (-1.43%)	⬇️
aiohttp/web_fileresponse.py	98.29% <0%> (+0.56%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b661900...8279f35. Read the comment docs.