Fixed issue with invoking _close() on closed event loop

[vladvildanov]

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Description of change

Closes #3431
Closes #3239

This PR fixes an issue when connection closed by garbage collector after event loop was closed. Also fixed an issue with __repr__ method tried to reference object already removed by garbage collector, by changing weak reference to strong reference.

[teodorfn]

LGTM. Tested and verified.

When using versions 5.0.2 to 5.2.0 i get RuntimeError: Event loop is closed (which was not an issue in versions <=5.0.1).

With this fix I don't get the error anymore. Thanks!

[akx]

This sort of change has me wonder why it was a weakref in the first place – probably for a reason?

[uglide]

Same concerns here as @akx. We need to ensure with a test that it doesn't create a circular reference and prevents garbage collecting unused SentinelConnectionPool objects.

[vladvildanov]

But isn't it already covered by current tests in test_sentinel.py? I mean, GC is always take in action after the tests been executed. I also had a doubts about this, but I didn't find any reason why weakref was used and current tests works as expected. Do you have a specific test case in mind that is worth to add?