Remove six and other 2.7 compatibility code

[cyberw]

Following up on #1120

[codecov]

Codecov Report

Merging #1253 into master will increase coverage by <.01%.
The diff coverage is 80.95%.

@@            Coverage Diff             @@
##           master    #1253      +/-   ##
==========================================
+ Coverage   77.66%   77.67%   +<.01%     
==========================================
  Files          20       20              
  Lines        2006     1980      -26     
  Branches      316      313       -3     
==========================================
- Hits         1558     1538      -20     
+ Misses        367      364       -3     
+ Partials       81       78       -3

Impacted Files	Coverage Δ
locust/util/deprecation.py	80% <0%> (+5%)	⬆️
locust/core.py	92.08% <100%> (-0.1%)	⬇️
locust/contrib/fasthttp.py	88.75% <100%> (+1.32%)	⬆️
locust/web.py	87.5% <100%> (-0.1%)	⬇️
locust/clients.py	91.39% <100%> (-0.1%)	⬇️
locust/main.py	32.91% <100%> (-0.15%)	⬇️
locust/runners.py	72.85% <60%> (-0.59%)	⬇️
locust/stats.py	85.71% <66.66%> (-0.07%)	⬇️
locust/inspectlocust.py	74.28% <66.66%> (-0.72%)	⬇️
... and 4 more

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