This can also happen in Intellij Ultimate, which has PyCharm integrated. The issue is as diagnosed above, you have the wrong interpreter selected.
The exact method to fix this for any given project is to go to Project Settings...Project and adjust the Project SDK. You can add a New Project SDK if you don't have Python 3 added by navigating to the python3 binary. This will fix the errors listed above. A shortcut to Project Settings is the blue checkerboard-type icon.
You can also add Python 3 as the default interpreter for Python projects. On OSX this is in File..Other Settings...Default Project Structure. There you can set the Project SDK which will now apply on each new project. It can be different on other platforms, but still similar.