Does graphene_sqlalchemy keep sessions open?

## tl;dr

Does `graphene_sqlalchemy` keep sessions open? I'm having the problem that Postgres doesnt run `drop_all()` in my test suite.

## Explanation

Hi, I have a fastAPI app and I'm using `graphene_sqlalchemy` to generate an API from database models defined on the declarative base of SQLAlchemy. The database is _postgres_. For testing I am running some pytests against the API while it's running. So, I start a throw-away postgres container, then I start the API app, then I run `pytest`.

The tests include a `reset_testdata()` function which also uses the SQLAlchemy ORM. It looks like:
```python
def reset_testdata():
close_all_sessions() # from sqlalchemy.orm.session
models.Base.metadata.drop_all(bind=engine)
models.Base.metadata.create_all(bind=engine)

db.add_all([
# ... add stuff
])

db.commit()
db.close()
```

I noticed, that once `reset_testdata()` is used, the pytest process hangs. No errors, not able to Ctrl+C, it just waits. I had this issue before and it usually stems from Postgres not allowing stuff like `drop_all()` if there are still active sessions. After some trying out I found that queries from my app (which are implemented by `graphene_sqlalchemy`) seem to keep a session open.
(I used this example in my app: https://docs.graphene-python.org/projects/sqlalchemy/en/latest/tutorial/#defining-our-models)

I tested this, and basically I can circumvent this problem by adding a middleware that closes all sessions after every request.
```python
@app.middleware('http')
async def close_sessions(request: Request, call_next):
try:
response = await call_next(request)
finally:
close_all_sessions()
return response
```

I wonder whether I am missing something here? Any experience with this issue (if it is even an issue)?