ci: Travis: test with Python 3.8

[blueyed]

No description provided.

[codecov]

Codecov Report

Merging #427 into master will increase coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
+ Coverage   71.02%   71.26%   +0.24%     
==========================================
  Files          32       32              
  Lines        2219     2255      +36     
  Branches      210      210              
==========================================
+ Hits         1576     1607      +31     
- Misses        586      591       +5     
  Partials       57       57

Impacted Files	Coverage Δ
pynvim/msgpack_rpc/event_loop/asyncio.py	59% <0%> (-5%)	⬇️
pynvim/api/window.py	100% <0%> (ø)	⬆️
pynvim/api/tabpage.py	100% <0%> (ø)	⬆️
pynvim/api/nvim.py	82.47% <0%> (+0.8%)	⬆️
pynvim/api/buffer.py	70.43% <0%> (+1.06%)	⬆️
test/conftest.py	75% <0%> (+1.66%)	⬆️
pynvim/plugin/script_host.py	25.84% <0%> (+2.14%)	⬆️

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