Move DummyCookieJar into cookiejar.py

[asvetlov]

Fix for #2242

[codecov-io]

Codecov Report

Merging #2244 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2244   +/-   ##
======================================
  Coverage    97.2%   97.2%           
======================================
  Files          39      39           
  Lines        7945    7945           
  Branches     1378    1378           
======================================
  Hits         7723    7723           
  Misses         98      98           
  Partials      124     124

Impacted Files	Coverage Δ
aiohttp/helpers.py	97.24% <ø> (-0.09%)	⬇️
aiohttp/cookiejar.py	100% <100%> (ø)	⬆️

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