[MAINT] add python3.8 to testing matrix

[jdkent]

Summary

Fixes #259.

List of changes proposed in this PR (pull-request)

• bump minimum versions of scikit-learn, numpy, and matplotlib to support 3.8
• add 3.8 to the travis testing matrix

[codecov]

Codecov Report

Merging #257 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #257   +/-   ##
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  Coverage    90.4%   90.4%           
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  Files          10      10           
  Lines         521     521           
  Branches       64      64           
======================================
  Hits          471     471           
  Misses         27      27           
  Partials       23      23