Fix implicit sign conversion warning (GH-4148)

Cython/Utility/ObjectHandling.c

@@ -2190,7 +2190,7 @@ bad:
 
 /////////////// PyObjectFastCall.proto ///////////////
 
-#define __Pyx_PyObject_FastCall(func, args, nargs)  __Pyx_PyObject_FastCallDict(func, args, nargs, NULL)
+#define __Pyx_PyObject_FastCall(func, args, nargs)  __Pyx_PyObject_FastCallDict(func, args, (size_t)(nargs), NULL)
 static CYTHON_INLINE PyObject* __Pyx_PyObject_FastCallDict(PyObject *func, PyObject **args, size_t nargs, PyObject *kwargs); /*proto*/
 
 /////////////// PyObjectFastCall ///////////////
@@ -2204,7 +2204,7 @@ static PyObject* __Pyx_PyObject_FastCall_fallback(PyObject *func, PyObject **arg
     PyObject *result;
     size_t i;
 
-    argstuple = PyTuple_New(nargs);
+    argstuple = PyTuple_New((Py_ssize_t)nargs);
     if (unlikely(!argstuple)) return NULL;
     for (i = 0; i < nargs; i++) {
         Py_INCREF(args[i]);