AttributeError: module 'open3d' has no attribute 'io'

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

system:ubuntu18.04
i use anaconda create env ,i try python version 3.6,3.7,3.8 and pip install difficult version open3d 0.10.0,0.15.1,0.11.0,0.11.1,but still have this problem.I also use commend: **conda install -c open3d-admin open3d** install open3d but cannot slove it .