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GH-41002: [Python] Remove pins for pytest-cython and conda-docs pytest (#45240)

### Rationale for this change

pytest-cython 0.3.1 was releases with the fix required for our tests to pass.

### What changes are included in this PR?

Remove unnecessary dependency pinned versions.

### Are these changes tested?

Via existing CI

### Are there any user-facing changes?

No
* GitHub Issue: #45239
* GitHub Issue: #41002

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>