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GH-40112: [CI][Python] Ensure CPython is selected, not PyPy (#40131)

Sometimes, mamba might select PyPy over CPython in certain environment upgrade/downgrade scenarios.

* Closes: #40112

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Uwe L. Korn <uwe.korn@quantco.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>