text: Title: [ENH] `SignatureTransformer` and `SignatureClassifier` `numpy 2` compatibility
  Body: `SignatureTransformer` and `SignatureClassifier` fail under `numpy 2`, this should be upgraded.
  
  The failure is
  
  ```
  >   from . import _tosig as tosig
  E   ImportError: numpy.core.multiarray failed to import
  ```