bpo-35059: PyObject_INIT() casts to PyObject* (GH-10674)

PyObject_INIT() and PyObject_INIT_VAR() now cast their first argument
to PyObject*, as done in Python 3.7.

Revert partially commit b4435e20a92af474f117b78b98ddc6f515363af5.
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index bed75ee..e4a4973 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -86,7 +86,7 @@
         op = (PyBytesObject *)PyObject_Malloc(PyBytesObject_SIZE + size);
     if (op == NULL)
         return PyErr_NoMemory();
-    (void)PyObject_INIT_VAR((PyVarObject *)op, &PyBytes_Type, size);
+    (void)PyObject_INIT_VAR(op, &PyBytes_Type, size);
     op->ob_shash = -1;
     if (!use_calloc)
         op->ob_sval[size] = '\0';
@@ -164,7 +164,7 @@
     op = (PyBytesObject *)PyObject_MALLOC(PyBytesObject_SIZE + size);
     if (op == NULL)
         return PyErr_NoMemory();
-    (void)PyObject_INIT_VAR((PyVarObject *)op, &PyBytes_Type, size);
+    (void)PyObject_INIT_VAR(op, &PyBytes_Type, size);
     op->ob_shash = -1;
     memcpy(op->ob_sval, str, size+1);
     /* share short strings */
@@ -1509,7 +1509,7 @@
     op = (PyBytesObject *)PyObject_MALLOC(PyBytesObject_SIZE + nbytes);
     if (op == NULL)
         return PyErr_NoMemory();
-    (void)PyObject_INIT_VAR((PyVarObject *)op, &PyBytes_Type, size);
+    (void)PyObject_INIT_VAR(op, &PyBytes_Type, size);
     op->ob_shash = -1;
     op->ob_sval[size] = '\0';
     if (Py_SIZE(a) == 1 && n > 0) {