add -> None to ClientSession ctor

[graingert]

What do these changes do?

This allows ClientSession to be constructed

Are there changes in behavior for the user?

Nope

Related issue number

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 news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[graingert]

Would be good to go though and add -> None to all ctors in this project

[codecov-io]

Codecov Report

Merging #3339 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3339   +/-   ##
=======================================
  Coverage   97.99%   97.99%           
=======================================
  Files          44       44           
  Lines        8039     8039           
  Branches     1357     1357           
=======================================
  Hits         7878     7878           
  Misses         67       67           
  Partials       94       94

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[asvetlov]

Would be good to go though and add -> None to all ctors in this project

Please do.
Thanks

[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].
[new issue]: https:/aio-libs/aiohttp/issues/new