bpo-38321: Fix _testcapimodule.c warning (GH-16494)

Fix the following warning:

    modules\_testcapimodule.c(6409):
    warning C4146: unary minus operator applied to unsigned type,
    result still unsigned

Modules/_testcapimodule.c

@@ -6406,7 +6406,7 @@ static PyType_Spec HeapCTypeWithDict_spec = {
 
 static struct PyMemberDef heapctypewithnegativedict_members[] = {
     {"dictobj", T_OBJECT, offsetof(HeapCTypeWithDictObject, dict)},
-    {"__dictoffset__", T_PYSSIZET, -sizeof(void*), READONLY},
+    {"__dictoffset__", T_PYSSIZET, -(Py_ssize_t)sizeof(void*), READONLY},
     {NULL} /* Sentinel */
 };