[MAINT] Reinstate open_spiel to Python 3.15 build in CI #1185

We have added Python 3.15 support in anticipation of the upcoming release, and this is being exercised in the CI.

At the moment open_spiel does not provide wheels for Python 3.15 for Windows, Building these takes a long time, far too long for a CI run.

Therefore in #1184 (2715027) we have temporarily disabled the tests of the demo notebook integrating with open_spiel on this OS/Python version combination.

This should be reinstated once there are prebuilt wheels available.