Add support for zpopmax and zpopmin redis commands (server 5.0.0+)

[leenr]

Available since Redis server 5.0.0.
https://redis.io/commands/zpopmax
https://redis.io/commands/zpopmin

[codecov]

Codecov Report

Merging #550 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #550      +/-   ##
==========================================
+ Coverage   96.85%   96.86%   +0.01%     
==========================================
  Files          57       57              
  Lines        8360     8396      +36     
  Branches      557      561       +4     
==========================================
+ Hits         8097     8133      +36     
  Misses        188      188              
  Partials       75       75

Impacted Files	Coverage Δ
aioredis/commands/sorted_set.py	100% <100%> (ø)	⬆️
tests/sorted_set_commands_test.py	100% <100%> (ø)	⬆️

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