gh-89546: Clean up PyType_FromMetaclass (GH-93686)

When changing PyType_FromMetaclass recently (GH-93012, GH-93466, GH-28748)
I found a bunch of opportunities to improve the code. Here they are.

Fixes: #89546

Automerge-Triggered-By: GH:encukou

Misc/NEWS.d/next/C API/2022-06-10-16-50-27.gh-issue-89546.mX1f10.rst

@@ -0,0 +1,4 @@
+:c:func:`PyType_FromMetaclass` (and other ``PyType_From*`` functions) now
+check that offsets and the base class's
+:c:member:`~PyTypeObject.tp_basicsize` fit in the new class's
+``tp_basicsize``.

Modules/_testcapimodule.c

@@ -1221,7 +1221,7 @@ static PyType_Spec MinimalMetaclass_spec = {
 
 static PyType_Spec MinimalType_spec = {
  .name = "_testcapi.MinimalSpecType",
- .basicsize = sizeof(PyObject),
+ .basicsize = 0, // Updated later
  .flags = Py_TPFLAGS_DEFAULT,
  .slots = empty_type_slots,
 };
@@ -1245,6 +1245,7 @@ test_from_spec_metatype_inheritance(PyObject *self, PyObject *Py_UNUSED(ignored)
  goto finally;
  }
 
+ MinimalType_spec.basicsize = (int)(((PyTypeObject*)class)->tp_basicsize);
  new = PyType_FromSpecWithBases(&MinimalType_spec, class);
  if (new == NULL) {
  goto finally;