requirements.txt: pin numpy to <2.0.0

[nilfm99]

Some of our python code is broken by the numpy v2.0.0 changes. Namely, printing numpy float values is now different:

• in numpy 1.x:

  >>> a = {"a": np.float64(3.2)}
  >>> print(a)
  {'a': 3.2}

• in numpy 2.x:

  >>> a = {"a": np.float64(3.2)}
  >>> print(a)
  {'a': np.float64(3.2)}

This doesn't play well with the ast.literal_eval-based parsing that we use. Longer term, we should get rid of ast.literal_eval and use JSON.