Replace PyObject_CallFunction() with fastcall

Replace
    PyObject_CallFunction(func, "O", arg)
and
    PyObject_CallFunction(func, "O", arg, NULL)
with
    _PyObject_CallArg1(func, arg)

Replace
    PyObject_CallFunction(func, NULL)
with
    _PyObject_CallNoArg(func)

_PyObject_CallNoArg() and _PyObject_CallArg1() are simpler and don't allocate
memory on the C stack.