Better checksum calculation

[asvetlov]

1. Use sha256 instead of md5
2. Put hashes into .hash directory to avoid working directory pollution.

[codecov-io]

Codecov Report

Merging #5183 into master will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5183      +/-   ##
==========================================
+ Coverage   97.45%   97.56%   +0.10%     
==========================================
  Files          43       43              
  Lines        8779     8779              
  Branches     1412     1412              
==========================================
+ Hits         8556     8565       +9     
+ Misses        112      100      -12     
- Partials      111      114       +3     

Flag	Coverage Δ
unit	97.56% <ø> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/helpers.py	96.69% <0.00%> (+0.23%)	⬆️
aiohttp/http_websocket.py	98.64% <0.00%> (+0.27%)	⬆️
aiohttp/http_parser.py	96.69% <0.00%> (+0.88%)	⬆️
aiohttp/http_writer.py	99.12% <0.00%> (+1.75%)	⬆️
aiohttp/frozenlist.py	97.91% <0.00%> (+2.08%)	⬆️

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[github-actions]

💚 Backport successful

The PR was backported to the following branches:

• ✅ 3.7
• ✅ 3.8