[trivial] update m2cgen to use NumPy 2.0's np.nan convention

NumPy 2.0 uses np.nan instead of np.NaN. Numpy 1.x supports both. Recommend switching to np.nan in the 3 places it is used in m2cgen, in tests/utils.py: 83: [np.nan] * self.X.shape[1], 107: [np.nan] * self.X.shape[1], 120: [np.nan] * self.X.shape[1], Figured I'd raise this issue for awareness, while I figure out how to properly upstream from our fork.