Update hscan command to return list of tuples

[popravich]

It was returning plain list of keys and values (['field-1', 'value-1', 'field-2', 'value-2'])
Now: [('field-1', 'value-1'), ('field-2', 'value-2')]

Similar to #334

[codecov]

Codecov Report

Merging #335 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #335      +/-   ##
==========================================
+ Coverage   96.77%   96.78%   +0.01%     
==========================================
  Files          55       55              
  Lines        7807     7802       -5     
  Branches      625      622       -3     
==========================================
- Hits         7555     7551       -4     
+ Misses        183      182       -1     
  Partials       69       69

Impacted Files	Coverage Δ
aioredis/util.py	95.2% <100%> (-0.36%)	⬇️
tests/hash_commands_test.py	100% <100%> (ø)	⬆️
aioredis/commands/hash.py	100% <100%> (ø)	⬆️
tests/sentinel_failover_test.py	88.72% <0%> (+0.75%)	⬆️

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