[3.6] Don't cancel web handler on disconnection (#4080)

[Jackenmen]

What do these changes do?

Backport #4080 to aiohttp 3.6. I'm not sure whether there's gonna be another 3.6 release or if 3.7 is still planned but this allows to handle connections issues properly so would be good to have.

#3804 wasn't backported to 3.6 so I had to put the try-finally block in the start() as _handle_request method doesn't exist here, I'm not entirely sure if I've done it properly. I could alternatively also backport #3804, either in this or seperate PR that this would depend on.

Are there changes in behavior for the user?

ConnectionResetError is raised on writing to closing transport instead of a warning.

Related issue number

#4080

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
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  • 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
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    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
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    • .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."

[asvetlov]

The time window for aiohttp 3.6 is closed. Let's see what can we do for 3.7