Cancel ping task on ConnectionResetError.

[jameshilliard]

What do these changes do?

Fixes:

  File "/usr/lib/python3.10/site-packages/aiohttp_sse/__init__.py", line 119, in wait
  File "/usr/lib/python3.10/site-packages/aiohttp_sse/__init__.py", line 157, in _ping
  File "/usr/lib/python3.10/site-packages/aiohttp/web_response.py", line 514, in write
  File "/usr/lib/python3.10/site-packages/aiohttp/http_writer.py", line 116, in write
  File "/usr/lib/python3.10/site-packages/aiohttp/http_writer.py", line 76, in _write
ConnectionResetError: Cannot write to closing transport

This issue appears to be related to #2989.

Are there changes in behavior for the user?

Calling await stream.wait() should no longer throw a ConnectionResetError if the client closes the connection unexpectedly.

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [x] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 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.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (06adced) 99.49% compared to head (4dbaceb) 99.53%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #370 +/- ## ========================================== + Coverage 99.49% 99.53% +0.03% ========================================== Files 4 4 Lines 394 427 +33 Branches 45 45 ========================================== + Hits 392 425 +33 Misses 1 1 Partials 1 1 ```

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