Add more type hints

[cyberw]

fixes #1260

[codecov]

Codecov Report

Merging #1582 into master will increase coverage by 0.47%.
The diff coverage is 87.87%.

@@            Coverage Diff             @@
##           master    #1582      +/-   ##
==========================================
+ Coverage   81.64%   82.12%   +0.47%     
==========================================
  Files          28       28              
  Lines        2594     2596       +2     
  Branches      395      395              
==========================================
+ Hits         2118     2132      +14     
+ Misses        378      367      -11     
+ Partials       98       97       -1     

Impacted Files	Coverage Δ
locust/contrib/fasthttp.py	85.84% <73.33%> (+0.47%)	⬆️
locust/env.py	96.82% <100.00%> (+0.10%)	⬆️
locust/shape.py	90.90% <100.00%> (+0.90%)	⬆️
locust/user/task.py	95.74% <100.00%> (+0.02%)	⬆️
locust/user/users.py	95.71% <100.00%> (-0.12%)	⬇️
locust/stats.py	90.08% <0.00%> (+0.20%)	⬆️
locust/runners.py	83.42% <0.00%> (+1.84%)	⬆️

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