Build CI Error on Python 3.7.6

Error at python3 -m coverage run test/XXX.py

Traceback (most recent call last):
  File "test/PyReplayBuffer.py", line 3, in <module>
    from cpprb import *
  File "/builds/ymd_h/cpprb/cpprb/__init__.py", line 1, in <module>
    from .PyReplayBuffer import (ReplayBuffer,PrioritizedReplayBuffer,
ModuleNotFoundError: No module named 'cpprb.PyReplayBuffer'