Run builds for python 3.9

[cyberw]

Had to wait for travis to support it...

[codecov]

Codecov Report

Merging #1607 into master will increase coverage by 1.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1607      +/-   ##
==========================================
+ Coverage   82.08%   83.45%   +1.36%     
==========================================
  Files          28       28              
  Lines        2596     3064     +468     
  Branches      395      561     +166     
==========================================
+ Hits         2131     2557     +426     
- Misses        367      408      +41     
- Partials       98       99       +1     

Impacted Files	Coverage Δ
locust/main.py	17.40% <0.00%> (-2.77%)	⬇️
locust/contrib/fasthttp.py	86.32% <0.00%> (+0.47%)	⬆️
locust/user/task.py	96.58% <0.00%> (+0.84%)	⬆️
locust/env.py	97.67% <0.00%> (+0.84%)	⬆️
locust/stats.py	90.78% <0.00%> (+0.90%)	⬆️
locust/web.py	94.51% <0.00%> (+2.04%)	⬆️
locust/argument_parser.py	80.00% <0.00%> (+2.80%)	⬆️
locust/runners.py	90.12% <0.00%> (+6.70%)	⬆️

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