Travis: test on Python 3.8-dev

[akx]

Refs #641

[codecov-io]

Codecov Report

Merging #642 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #642      +/-   ##
==========================================
+ Coverage   90.42%   90.54%   +0.12%     
==========================================
  Files          24       24              
  Lines        4083     4135      +52     
==========================================
+ Hits         3692     3744      +52     
  Misses        391      391

Impacted Files	Coverage Δ
babel/plural.py	96.07% <0%> (+0.02%)	⬆️
babel/dates.py	91.6% <0%> (+0.04%)	⬆️
babel/support.py	82.74% <0%> (+0.06%)	⬆️
babel/messages/catalog.py	94.79% <0%> (+0.08%)	⬆️
babel/core.py	96.04% <0%> (+0.53%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 88fe29b...ccf4001. Read the comment docs.