First make sure that you are not trying to import the module after activating the virtualenv. If you have your virtualenv activated and didn't supply the argument
--system-site-packages while creating it, the module won't be available to you to import in virtualenv.
If you are not using a virtualenv and still unable to import the module, then make sure that you are using the Python 3 interpreter and not the Python 2 interpreter. Since you installed the package for python3, it won't be available to import in python2 interpreter. Next try to purge and reinstall the module using:
sudo apt-get purge python3-dateutil
sudo apt-get install python3-dateutil
Purging the package and reinstalling it should fix any issues which was restricting to import the module.