[ci] Add Python 3.8 testing in Travis CI

[teojgo]

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[codecov-io]

Codecov Report

Merging #1118 into master will increase coverage by 0.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1118      +/-   ##
==========================================
+ Coverage   91.61%   91.89%   +0.28%     
==========================================
  Files          81       81              
  Lines       11236    11626     +390     
==========================================
+ Hits        10294    10684     +390     
  Misses        942      942

Impacted Files	Coverage Δ
reframe/core/containers.py	100% <0%> (ø)	⬆️
unittests/test_runtime.py	100% <0%> (ø)	⬆️
unittests/test_check_filters.py	100% <0%> (ø)	⬆️
unittests/test_sanity_functions.py	100% <0%> (ø)	⬆️
reframe/utility/color.py	100% <0%> (ø)	⬆️
unittests/test_deferrable.py	99.62% <0%> (ø)	⬆️
unittests/test_buildsystems.py	99.16% <0%> (ø)	⬆️
reframe/core/buildsystems.py	97.11% <0%> (+0.01%)	⬆️
unittests/test_policies.py	98.93% <0%> (+0.02%)	⬆️
unittests/test_cli.py	93.33% <0%> (+0.06%)	⬆️
... and 26 more

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