Update TensorFlow/Keras legacy APIs, test Python 3.12

[mrogowski]

• TensorFlow 2.16.1 uses Keras 3.0 as the default. It seems that the set_session function which was used in PySPOD does not exist anymore.
• Fixes: ValueError: When using `save_weights_only=True` in `ModelCheckpoint`, the filepath provided must end in `.weights.h5` (Keras weights format).` Received: filepath=[...]/real0__weights.h5
• We were previously not testing Python 3.12 because there was no TensorFlow for Python 3.12 on PyPI. They have since added it.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.72%. Comparing base (58286ad) to head (d4a3053).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #47 +/- ## ========================================== + Coverage 76.57% 76.72% +0.14% ========================================== Files 16 16 Lines 3531 3532 +1 Branches 478 554 +76 ========================================== + Hits 2704 2710 +6 + Misses 669 664 -5 Partials 158 158 ```

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