Enable Python 3.8 support

[jklaise]

No description provided.

[codecov]

Codecov Report

Merging #280 into master will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   83.27%   83.52%   +0.24%     
==========================================
  Files          55       55              
  Lines        6506     6602      +96     
==========================================
+ Hits         5418     5514      +96     
  Misses       1088     1088              

Impacted Files	Coverage Δ
alibi/explainers/tests/test_ale.py	100.00% <100.00%> (ø)
alibi/explainers/tests/test_anchor_base.py	100.00% <100.00%> (ø)
alibi/explainers/tests/test_anchor_tabular.py	28.07% <100.00%> (+1.72%)	⬆️
alibi/explainers/tests/test_anchor_text.py	100.00% <100.00%> (ø)
alibi/tests/test_utils.py	100.00% <0.00%> (ø)
alibi/utils/tests/test_data.py	100.00% <0.00%> (ø)
alibi/explainers/tests/test_cem.py	100.00% <0.00%> (ø)
alibi/utils/tests/test_distance.py	100.00% <0.00%> (ø)
alibi/utils/tests/test_gradients.py	100.00% <0.00%> (ø)
... and 16 more