title: sqlalchemy.exc.ArgumentError: Can't load plugin: sqlalchemy.dialects:driver
body: <p>I am trying to run <code>alembic</code> migration and when I run </p>
  
  <pre><code>alembic revision --autogenerate -m "Added initial tables"
  </code></pre>
  
  <p>It fails saying</p>
  
  <pre><code>sqlalchemy.exc.ArgumentError: Can't load plugin: sqlalchemy.dialects:driver
  </code></pre>
  
  <p>the database url is </p>
  
  <pre><code>postgresql+psycopg2://dev:passwd@localhost/db
  </code></pre>
  
  <p>and I even have <code>psycopg2</code> installed in my virtualenv</p>
  
  <pre><code>$yolk -l
  Flask-Login     - 0.1.3        - active
  Flask-SQLAlchemy - 0.16         - active
  Flask           - 0.9          - active
  Jinja2          - 2.6          - active
  Mako            - 0.7.3        - active
  MarkupSafe      - 0.15         - active
  Python          - 2.7.2        - active development (/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload)
  SQLAlchemy      - 0.8.0        - active
  Werkzeug        - 0.8.3        - active
  alembic         - 0.4.2        - active
  antiorm         - 1.1.1        - active
  appscript       - 1.0.1        - active
  distribute      - 0.6.27       - active
  envoy           - 0.0.2        - active
  osascript       - 0.0.4        - active
  pep8            - 1.4.5        - active
  pip             - 1.1          - active
  psycopg2        - 2.4.6        - active
  wsgiref         - 0.1.2        - active development (/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7)
  yolk            - 0.4.3        - active
  </code></pre>
  
  <p>Whay could be causing this issue?</p>