Import error: ModuleNotFoundError: No module named 'flatland.envs'

Hi,

I installed flatland-rl succedfully from pip. But when I import it, there is an error:

>>> from flatland.envs.generators import complex_rail_generator
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'flatland.envs'
>>> import flatland
>>> 

So, I can import flatland but cannot import flatland.envs? I don't know how to fix it.

My computer: OS: Ubuntu 16.04 I use miniconda to create new environment Python 3.6