ModuleNotFoundError: No module named 'mdtraj.utils.six'

[LuhuanWu]

Hi thank you for developing this library!

I was trying to test this package using pytest but failed. The error I got is ModuleNotFoundError: No module named 'mdtraj.utils.six'
when running this line
from mdtraj.utils.six import PY3 in the bgmol.tpl.hdf5 file.

This issue can be fixed by changing mdtraj.utils.six to the regular six library. I wonder why this issue happened, and was it because you have an internal version of mdtraj?