Add DummyCookieJar helper

[alefteris]

What do these changes do?

Adds a DummyCookieJar class in the helpers module. It's a no-op cookie storage that can be used with ClientSession when cookie processing is not needed.

Are there changes in behavior for the user?

No.

Related issue number

There was an issue comment that encouraged this contribution.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new entry to CHANGES.rst
  • Choose any open position to avoid merge conflicts with other PRs.
  • Add a link to the issue you are fixing (if any) using #issue_number format at the end of changelog message. Use Pull Request number if there are no issues for PR or PR covers the issue only partially.

[codecov-io]

Codecov Report

Merging #1830 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1830      +/-   ##
==========================================
+ Coverage   97.23%   97.23%   +<.01%     
==========================================
  Files          37       37              
  Lines        7482     7496      +14     
  Branches     1300     1300              
==========================================
+ Hits         7275     7289      +14     
  Misses         87       87              
  Partials      120      120

Impacted Files	Coverage Δ
aiohttp/helpers.py	97.66% <100%> (+0.07%)	⬆️

---

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 8d8634a...659550b. Read the comment docs.

[fafhrd91]

great!

lgtm. @kxepal @asvetlov any comments?

[kxepal]

Good one

[lock]

This thread has been automatically locked since there has not been
any recent activity after it was closed. Please open a new issue for
related bugs.

If you feel like there's important points made in this discussion,
please include those exceprts into that new issue.