Black format check result inconsistent between different Black versions

[HiromuHota]

Describe the bug

Black format check result is inconsistent between 18.9b0+ and 18.6b3 as described below.

$ black --version
black, version 18.9b0
$ black src/ --diff
All done! ✨ 🍰 ✨
90 files left unchanged.


$ black --version
black, version 18.6b3
$ black src/ --diff
--- src/fonduer/learning/__init__.py	2018-12-11 22:13:08.808439 +0000
+++ src/fonduer/learning/__init__.py	2019-03-22 16:27:11.950875 +0000
@@ -1,9 +1,9 @@
 from fonduer.learning.disc_models.logistic_regression import LogisticRegression
 from fonduer.learning.disc_models.lstm import LSTM
 from fonduer.learning.disc_models.sparse_logistic_regression import (
-    SparseLogisticRegression,
+    SparseLogisticRegression
 )
 from fonduer.learning.disc_models.sparse_lstm import SparseLSTM
 
 __all__ = ["LogisticRegression", "LSTM", "SparseLogisticRegression", "SparseLSTM"]
 
reformatted src/fonduer/learning/__init__.py
All done! ✨ 🍰 ✨
1 file reformatted, 89 files left unchanged.

To Reproduce
Steps to reproduce the behavior:

See above.

Expected behavior

A solution is either

1. Reformat src/fonduer/learning/__init__.py
2. Make sure black is 18.9b0+ by defining so in requirements-dev.txt

Error Logs/Screenshots
If applicable, add error logs or screenshots to help explain your problem.

See above.

Environment (please complete the following information):

• OS: macOS (Mojave)
• PostgreSQL Version: N/A
• Poppler Utils Version: N/A
• Fonduer Version: 138e4be

Additional context
Add any other context about the problem here.

[HiromuHota]

The root cause is the inconsistent formatting between isort and black (18.6b3) (psf/black#250).
Since psf/black#504 (i.e., Black 18.9b0+), black became consistent with isort.
Therefore, we should make sure to use black 18.9b0+. Let me submit a PR.