Add support for TensorFlow 2.16 / Keras 3.

- Apply some minor backward changes (and, when required, branches) to
  support breaking changes in Keras 3 (the default for TensorFlow 2.16
  and onwards), while preserving support for older versions.
- On the side, split a test about 'Model' serialization in two tests,
  one for config serializability, the other for instantiation from a
  non-serialized config dict.
- The current code has been tested (by running all unit tests) with
  both TensorFlow 2.11 and 2.16, in a Python 3.10 environment.