Update main.py about gevent.signal()

[test-bai-cpu]

For update in gevent 1.5.0(gevent/gevent#1529), the usage of gevent.signal() should also be updated.

[codecov]

Codecov Report

Merging #1326 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1326   +/-   ##
=======================================
  Coverage   80.72%   80.72%           
=======================================
  Files          24       24           
  Lines        2184     2184           
  Branches      334      334           
=======================================
  Hits         1763     1763           
  Misses        333      333           
  Partials       88       88           

Impacted Files	Coverage Δ
locust/main.py	21.57% <0.00%> (ø)

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[heyman]

Thanks!