SequentialTaskSet improvements

[cyberw]

Clarify documentation to indicate you can do multiple requests in a task without a SequentialTask.

Support task weights (repetitions) instead of silently ignoring them.

[codecov]

Codecov Report

Merging #1579 into master will increase coverage by 0.32%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1579      +/-   ##
==========================================
+ Coverage   81.80%   82.12%   +0.32%     
==========================================
  Files          28       28              
  Lines        2583     2585       +2     
  Branches      393      394       +1     
==========================================
+ Hits         2113     2123      +10     
+ Misses        375      367       -8     
  Partials       95       95              

Impacted Files	Coverage Δ
locust/user/sequential_taskset.py	86.20% <100.00%> (+1.02%)	⬆️
locust/user/task.py	94.65% <100.00%> (+0.53%)	⬆️
locust/runners.py	83.76% <0.00%> (+1.29%)	⬆️

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