Remove six usage

[cedy]

Which problem is this PR solving?

309

Short description of the changes

Removes six usage across the project, except jaeger_client/thrift_gen/ folder.

[codecov[bot]]

Codecov Report

Merging #310 (00dcb7b) into master (ec2ed1c) will increase coverage by 0.02%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
+ Coverage   95.30%   95.33%   +0.02%     
==========================================
  Files          23       23              
  Lines        1940     1931       -9     
  Branches      273      272       -1     
==========================================
- Hits         1849     1841       -8     
  Misses         56       56              
+ Partials       35       34       -1     

Impacted Files	Coverage Δ
jaeger_client/utils.py	90.16% <ø> (-0.16%)	:arrow_down:
jaeger_client/codecs.py	97.00% <84.61%> (-0.02%)	:arrow_down:
jaeger_client/metrics/metrics.py	100.00% <100.00%> (ø)
jaeger_client/sampler.py	100.00% <100.00%> (ø)
jaeger_client/span.py	96.55% <100.00%> (-0.03%)	:arrow_down:
jaeger_client/thrift.py	98.03% <100.00%> (+0.89%)	:arrow_up:
jaeger_client/tracer.py	99.34% <100.00%> (-0.01%)	:arrow_down:

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[yurishkuro]

Thanks!