temp_dpool fixture should be global

## Describe the bug

Right now, the `temp_dpool` fixture is limited to `test_dataobject`, which means any other test file that wants to create `dataobjects` needs to maintain unique names for their classes otherwise they will face name collisions.

If the `temp_dpool` fixture is made global, then any test that wants to test using a `dataobject` can just use that fixture.

## Platform

Please provide details about the environment you are using, including the following:

- Interpreter version: 0.9.x
- Library version: 3.9