Don't cancel self (v2)

[Olegt0rr]

What do these changes do?

https://github.com/aio-libs/aiohttp-sse/pull/456#pullrequestreview-1860917238

Are there changes in behavior for the user?

Adds some warnings if EventSource was used wrong way

Related issue number

no issue

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes

[codecov[bot]]

Codecov Report

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

Comparison is base (7fdb432) 100.00% compared to head (26fa6a0) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #457 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 4 4 Lines 452 458 +6 Branches 52 53 +1 ========================================= + Hits 452 458 +6 ```

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[Olegt0rr]

I'll make another PR with cancel swallowing