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

[ssundaresh-thirdpartywatcher]

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.