Merge pull request #28 from wesselb/wesselb/fix-jax-tensor-faithful

Force `_jax_tensor` to be faithful

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Simplify `B.mean` for PyTorch (closes #21)

Fix support for custom JAX operations

Don't force tests to run with NumPy 2

Fix `device` for JAX

Add fdm as test dependency

Fix `keras.KerasTensor` availability issue

Fix Python 3.8

Merge branch 'master' of github.com:wesselb/lab