Add python38 in the pipeline

[argaen]

No description provided.

[argaen]

Two tests failing
https://travis-ci.org/argaen/aiocache/jobs/565782461

[codecov]

Codecov Report

Merging #458 into master will decrease coverage by 0.1%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
- Coverage   99.58%   99.48%   -0.11%     
==========================================
  Files          11       11              
  Lines         967      970       +3     
  Branches      104      105       +1     
==========================================
+ Hits          963      965       +2     
- Misses          4        5       +1

Impacted Files	Coverage Δ
aiocache/serializers/serializers.py	95.08% <60%> (-1.47%)	⬇️

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