Fix static analysis of scoped_session

Not sure why the current code was written this way, but it failed to make pylint "find" scoped_session methods.

Here is a little example, in a virtualenv with sqlalchemy and pylint installed:
```
from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.orm import sessionmaker, scoped_session
from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()


class SomeClass(Base):

    __tablename__ = 'some_table'
    key = Column(Integer, primary_key=True)
    name = Column(String(50))


if __name__ == '__main__':
    engine = create_engine('sqlite:///:memory:', echo=True)
    session_factory = sessionmaker(bind=engine)
    Session = scoped_session(session_factory)
    Session.query(SomeClass).get('random')
```

pylint will return:
```
pylint --report=n --disable=C0111,C0103 test.py
************* Module test
W:  9, 0: Class has no __init__ method (no-init)
R:  9, 0: Too few public methods (0/2) (too-few-public-methods)
E: 20, 4: Instance of 'scoped_session' has no 'query' member (no-member)
```

With this patch applied, the error disappears as expected. Since I use a lot of scoped_session in my code, it is really annoying to get this error and it may hide real errors if I disable it.

Thanks for considering.