using pip3: module "importlib._bootstrap" has no attribute "SourceFileLoader"

I have installed pip for python 3.6 on Ubuntu 14. After I run sudo apt-get install python3-pip to install pip3, it works very well. However, after installation, when I am trying to run pip3 install packagename to install a new package, something strange occurs: File "/usr/lib/python3/dist-packages/pkg_resources.py", line 1479, in <module> register_loader-type(importlib_bootstrap.SourceFileLoader, DefaultProvider) AttributeError: module "importlib._bootstrap" has no attribute "SourceFileLoader" It seems that I did nothing wrong, and I really cannot figure out the reason.