Add Python 3.8 to the test matrix

[jdufresne]

Python 3.8 was released on October 14th, 2019.

[codecov]

Codecov Report

Merging #1110 into master will increase coverage by 0.7%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #1110     +/-   ##
=========================================
+ Coverage   72.45%   73.15%   +0.7%     
=========================================
  Files          17       17             
  Lines        1706     1751     +45     
  Branches      255      255             
=========================================
+ Hits         1236     1281     +45     
  Misses        408      408             
  Partials       62       62

Impacted Files	Coverage Δ
locust/rpc/zmqrpc.py	100% <0%> (ø)	⬆️
locust/rpc/protocol.py	100% <0%> (ø)	⬆️
locust/contrib/fasthttp.py	92.94% <0%> (+0.12%)	⬆️
locust/core.py	85.26% <0%> (+0.33%)	⬆️
locust/stats.py	81.44% <0%> (+0.64%)	⬆️
locust/runners.py	55.42% <0%> (+1.17%)	⬆️
locust/web.py	82.75% <0%> (+1.62%)	⬆️

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[heyman]

Nice!