[3.7] Fix python 3.8 tests under Windows (#4513)

[asvetlov]

No description provided.

[codecov-io]

Codecov Report

Merging #4514 into 3.7 will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              3.7    #4514      +/-   ##
==========================================
+ Coverage   97.87%   97.93%   +0.05%     
==========================================
  Files          44       44              
  Lines        8911     8945      +34     
  Branches     1395     1406      +11     
==========================================
+ Hits         8722     8760      +38     
+ Misses         79       78       -1     
+ Partials      110      107       -3

Impacted Files	Coverage Δ
aiohttp/payload_streamer.py	100% <100%> (ø)	⬆️
aiohttp/tracing.py	100% <0%> (ø)	⬆️
aiohttp/cookiejar.py	100% <0%> (ø)	⬆️
aiohttp/web_request.py	97.52% <0%> (ø)	⬆️
aiohttp/helpers.py	96.5% <0%> (+0.01%)	⬆️
aiohttp/client_reqrep.py	97.48% <0%> (+0.01%)	⬆️
aiohttp/frozenlist.py	97.91% <0%> (+0.04%)	⬆️
aiohttp/pytest_plugin.py	97.57% <0%> (+0.13%)	⬆️
aiohttp/web_response.py	98.2% <0%> (+0.22%)	⬆️
aiohttp/web_fileresponse.py	98.29% <0%> (+0.56%)	⬆️
... and 1 more

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