Suppress "Future exception was never retrieved"

[Pliner]

The fix suppresses the following error via ugly hack using .add_done_callback(lambda f: f.exception()):

Future exception was never retrieved
future: <Future finished exception=OperationalError('Connection closed')>

Also I had to add loop.is_closed() to suppress this error in tests:

Traceback (most recent call last):
  File "/Users/yurypliner/Sources/aiopg/aiopg/pool.py", line 476, in __del__
    conn.close()
  File "/Users/yurypliner/Sources/aiopg/aiopg/connection.py", line 995, in close
    self._close()
  File "/Users/yurypliner/Sources/aiopg/aiopg/connection.py", line 990, in _close
    self._notifies_proxy.close(
  File "/Users/yurypliner/Sources/aiopg/aiopg/utils.py", line 152, in close
    self._close_event.set_exception(exception)
  File "/Users/yurypliner/.pyenv/versions/3.9.2/lib/python3.9/asyncio/base_events.py", line 746, in call_soon
    self._check_closed()
  File "/Users/yurypliner/.pyenv/versions/3.9.2/lib/python3.9/asyncio/base_events.py", line 510, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

[codecov[bot]]

Codecov Report

Merging #862 (6814434) into master (6f9958b) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #862   +/-   ##
=======================================
  Coverage   93.42%   93.43%           
=======================================
  Files          11       11           
  Lines        1566     1568    +2     
  Branches      184      186    +2     
=======================================
+ Hits         1463     1465    +2     
  Misses         72       72           
  Partials       31       31           

Impacted Files	Coverage Δ
aiopg/connection.py	95.84% <100.00%> (+<0.01%)	:arrow_up:
aiopg/utils.py	91.42% <100.00%> (+0.08%)	:arrow_up:

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