Fix implicit signedness implicit conversion warning
This is in a header, to produces the following warning in any source file that includes the header:
VTK/Wrapping/PythonCore/vtkPythonArgs.h:948:39: warning: implicit conversion changes signedness: 'size_t' (aka 'unsigned long') to 'Py_ssize_t' (aka 'long') [-Wsign-conversion]