Update sorted set commands to return list[tuple] instead of plain list.

[popravich]

Amend sorted set commands to return pairs instead of plain list when withscores argument is set to True (resolves #209)

[codecov]

Codecov Report

Merging #334 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
+ Coverage   96.71%   96.75%   +0.04%     
==========================================
  Files          55       55              
  Lines        7818     7807      -11     
  Branches      625      625              
==========================================
- Hits         7561     7554       -7     
+ Misses        184      183       -1     
+ Partials       73       70       -3

Impacted Files	Coverage Δ
aioredis/commands/sorted_set.py	100% <100%> (ø)	⬆️
tests/sorted_set_commands_test.py	100% <100%> (ø)	⬆️
aioredis/commands/pubsub.py	95.65% <0%> (+1.08%)	⬆️
tests/sentinel_failover_test.py	87.21% <0%> (+1.5%)	⬆️

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