Passing expire_on_commit=False on sessionmaker() [see #181]

Accessing db objects outside of their session seems to fail on SQLAlchemy >= 1.4
with a `Instance `Instance <x> is not bound to a Session` error.

Setting expire_on_commit=False on the session seems to somehow fix the issue
(see https://stackoverflow.com/questions/3039567/sqlalchemy-detachedinstanceerror-with-regular-attribute-not-a-relation)

platypush/user/__init__.py

@@ -234,7 +234,7 @@ class UserManager:
 
     def _get_db_session(self):
         Base.metadata.create_all(self._engine)
-        session = scoped_session(sessionmaker())
+        session = scoped_session(sessionmaker(expire_on_commit=False))
         session.configure(bind=self._engine)
         return session()