[ci] allow tests to fail on python 3.8

[enkidulan]

fix travis configuration

[codecov-io]

Codecov Report

Merging #199 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   77.02%   77.02%           
=======================================
  Files         132      132           
  Lines        5823     5823           
  Branches      631      631           
=======================================
  Hits         4485     4485           
  Misses       1097     1097           
  Partials      241      241

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