Avoid creating redundant query object

[amos402]

Since motor_cursor would create a query object every time when it is accessed, this change it's for preventing that meaningless object allocation.

[roman-right]

Sounds good.
But it looks like mypy doesn't agree with something. You can run it against your code on committing using pre-commit: pre-commit install

[amos402]

The first commit was for not to breaking any logic changes.
Yet, the more delightful solution is to set the cursor in __aiter__, now it should satisfied the mypy.
Now it can be called by aiter multiple times which match the generator standard.