use six library for isinstance

[ashtul]

fix #64

[codecov]

Codecov Report

Merging #66 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #66   +/-   ##
=======================================
  Coverage   90.55%   90.55%           
=======================================
  Files          12       12           
  Lines        1493     1493           
=======================================
  Hits         1352     1352           
  Misses        141      141           

Impacted Files	Coverage Δ
redisearch/_util.py	85.71% <100.00%> (ø)
redisearch/client.py	86.02% <100.00%> (ø)

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[ashtul]

@quintonparker can you confirm this fixed the issue

[quintonparker]

@ashtul works on my machine ™️

😄