Fix subplot args convert

[Sologala]

Unproperly args converting in void subplot(long , long , long) may cause segment fault.

https://github.com/lava/matplotlib-cpp/blob/2acb4d07d263f04363a5156c28e85c142345792b/matplotlibcpp.h#L2252-L2267

[SirTwinkleberry]

The implicit conversion part is a duplicate of #333

[ArterioRodrigues]

Would double conversion also cause a seg fault? I am using this it seems to work.

 PyObject* args = PyTuple_New(3); 
     PyTuple_SetItem(args, 0, PyDouble_FromLong(nrows)); 
     PyTuple_SetItem(args, 1, PyDouble_FromLong(ncols)); 
     PyTuple_SetItem(args, 2, PyDouble_FromLong(plot_number));