Fix errors due to the major update of NumPy

[KeishiS]

Thank you for sharing graph kernels implementation. I fixed 2 errors caused by the update of numpy.

• Some older numpy data types are used in GKextCPy_wrap.cxx. However, such older types are removed due to major update of numpy 2.0. I restricted the installed numpy version to be below 2.0.
• From numpy 1.15 to 1.16, the default value of load function was changed. I modified the code to explicitly specify the argument.