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GH-35014: [Python] Make sure unit tests can run without acero (#35017)

### Rationale for this change

Fixes two failing nightlies:

 * [example-python-minimal-build-fedora-conda](https://github.com/ursacomputing/crossbow/actions/runs/4653275808/jobs/8234009627)
 * [example-python-minimal-build-ubuntu-venv](https://github.com/ursacomputing/crossbow/actions/runs/4653280100/jobs/8234017973)

### What changes are included in this PR?

Adds a pytest mark for Acero, since it is now required.

### Are these changes tested?

Yes, this fixes existing tests. Validated it works locally.

### Are there any user-facing changes?

No.
* Closes: #35014

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>