title: Importing flask.ext raises ModuleNotFoundError
body: <p>When I try to use Flask extensions, my application is raising <code>ModuleNotFound</code> errors on lines like <code>from flask.ext.sqlalchemy import SQLAlchemy</code> or <code>from flask.exthook import ExtDeprecationWarning</code>. I have Flask and the extension installed. Why do I get this error?</p>