Remove python2-only __nonzero__ method from cluster.py

[sobolevn]

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Description of change

__nonzero__ is not used in python3. And since currenct setup.py specifies that >= 3.7 is required, there's no need for this method.

[codecov-commenter]

Codecov Report

Merging #2313 (3a85387) into master (c94821c) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2313      +/-   ##
==========================================
- Coverage   92.16%   92.16%   -0.01%     
==========================================
  Files         110      110              
  Lines       28642    28640       -2     
==========================================
- Hits        26399    26397       -2     
  Misses       2243     2243              

Impacted Files	Coverage Δ
redis/cluster.py	90.47% <ø> (+0.10%)	⬆️
redis/asyncio/client.py	92.09% <0.00%> (-0.18%)	⬇️
tests/test_cluster.py	96.96% <0.00%> (ø)

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