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MINOR: [Python][CI] Add upper bound on pytest version (#39827)

### Rationale for this change

The PyArrow test suite relies on the pytest-lazy-fixture plugin, which breaks on pytest 8.0.0: https://github.com/TvoroG/pytest-lazy-fixture/issues/65

### What changes are included in this PR?

Avoid installing pytest 8 on CI builds, by putting an upper bound on the pytest version.

### Are these changes tested?

Yes, by construction.

### Are there any user-facing changes?

No.

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>