Fix LGTM.com alerts

[popravich]

What do these changes do?

Fix LGTM.com alerts https://lgtm.com/projects/g/aio-libs/aioredis/alerts/?mode=list

Are there changes in behavior for the user?

no

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES/ folder

[lgtm-com]

This pull request fixes 8 alerts when merging 3cba4dd into bac9b42 - view on LGTM.com

fixed alerts:

• 4 for Unnecessary 'else' clause in loop
• 2 for Module is imported with 'import' and 'import from'
• 1 for Unused local variable
• 1 for Mismatch between signature and use of an overridden method

[codecov]

Codecov Report

Merging #646 into master will increase coverage by 0.48%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
+ Coverage   96.85%   97.34%   +0.48%     
==========================================
  Files          57       57              
  Lines        8551     9102     +551     
  Branches      578      640      +62     
==========================================
+ Hits         8282     8860     +578     
+ Misses        192      170      -22     
+ Partials       77       72       -5

Impacted Files	Coverage Δ
aioredis/sentinel/pool.py	81.27% <100%> (+4.49%)	⬆️
aioredis/abc.py	100% <100%> (ø)	⬆️
aioredis/locks.py	95.83% <100%> (-0.17%)	⬇️
tests/stream_commands_test.py	100% <0%> (ø)	⬆️
tests/server_commands_test.py	100% <0%> (ø)	⬆️
tests/sentinel_commands_test.py	100% <0%> (ø)	⬆️
tests/generic_commands_test.py	100% <0%> (ø)	⬆️
tests/pool_test.py	99.77% <0%> (+0.04%)	⬆️
tests/pubsub_receiver_test.py	99.67% <0%> (+0.81%)	⬆️
tests/_testutils.py	85.48% <0%> (+1.27%)	⬆️
... and 4 more

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