Adopt to isort>=5.0.0

[senwu]

Description of the problems or issues

Adopt to isort v5.0.0

Test plan

No test plan

Checklist

• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [ ] I have updated the CHANGELOG.rst accordingly.

[codecov[bot]]

Codecov Report

Merging #75 into master will decrease coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   88.90%   88.86%   -0.04%     
==========================================
  Files          38       38              
  Lines        1640     1635       -5     
  Branches      329      329              
==========================================
- Hits         1458     1453       -5     
  Misses         94       94              
  Partials       88       88              

Flag	Coverage Δ
#unittests	88.86% <100.00%> (-0.04%)	:arrow_down:

Impacted Files	Coverage Δ
src/emmental/learner.py	79.91% <100.00%> (ø)
src/emmental/model.py	79.91% <100.00%> (-0.09%)	:arrow_down:
src/emmental/modules/embedding_module.py	98.24% <100.00%> (-0.04%)	:arrow_down:
src/emmental/modules/identity_module.py	100.00% <100.00%> (ø)
src/emmental/modules/rnn_module.py	100.00% <100.00%> (ø)
src/emmental/modules/sparse_linear_module.py	96.00% <100.00%> (-0.16%)	:arrow_down: