Support Python 3.8

[ogrisel]

Fixes #927 to support Python 3.8.

[codecov]

Codecov Report

Merging #936 into master will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #936      +/-   ##
==========================================
- Coverage    95.4%   95.38%   -0.02%     
==========================================
  Files          45       45              
  Lines        6460     6462       +2     
==========================================
+ Hits         6163     6164       +1     
- Misses        297      298       +1

Impacted Files	Coverage Δ
joblib/hashing.py	93.16% <100%> (ø)	⬆️
joblib/test/test_module.py	94.44% <50%> (-5.56%)	⬇️
joblib/test/test_store_backends.py	91.17% <0%> (-5.89%)	⬇️
joblib/_parallel_backends.py	93.6% <0%> (-0.81%)	⬇️
joblib/disk.py	88.33% <0%> (+6.66%)	⬆️

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[ogrisel]

The coverage drop is expected as long as we don't have a Python 3.8 CI but we need to wait for the 3.8 release first...