Check code coverage in tests-python37 on CircleCI.

[hvy]

Motivation

Aims to reduce the job queue on CircleCI. See #1343.

Description of the changes

Check the code coverage in tests-python37 instead of creating a separate job sweeping over all unit tests.

[codecov-commenter]

Codecov Report

Merging #1348 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1348      +/-   ##
==========================================
- Coverage   87.87%   87.85%   -0.02%     
==========================================
  Files          98       98              
  Lines        7348     7339       -9     
==========================================
- Hits         6457     6448       -9     
  Misses        891      891              

Impacted Files	Coverage Δ
optuna/distributions.py	93.68% <0.00%> (-0.31%)	⬇️
optuna/storages/redis.py	94.13% <0.00%> (-0.06%)	⬇️
optuna/storages/rdb/storage.py	93.66% <0.00%> (-0.04%)	⬇️
optuna/storages/in_memory.py	97.66% <0.00%> (-0.03%)	⬇️
optuna/storages/cached_storage.py	96.44% <0.00%> (-0.02%)	⬇️
optuna/storages/base.py	67.59% <0.00%> (ø)
optuna/trial/_trial.py	92.10% <0.00%> (+0.33%)	⬆️

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[toshihikoyanase]

LGTM!

[crcrpar]

Looks solid.