Issue title: ModuleNotFoundError: No module named 'motmetrics.math_util'
 Issue body: i install the motmetrics==1.1.3, but no module named 'motmetrics.math_util'. which version have the module?

 Comments: 
 Comment 0: Can you try the latest version of motmetrics?

 Comment 1: > Can you try the latest version of motmetrics?

thanks, i just solved it by https://github.com/cheind/py-motmetrics
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