Clarify C calling convention for __class_getitem__ in PEP 560 (#643)

pep-0560.rst

@@ -206,6 +206,59 @@ the backwards compatibility::
       return meta(name, resolved_bases, ns, **kwds)
 
 
+Using ``__class_getitem__`` in C extensions
+-------------------------------------------
+
+As mentioned above, ``__class_getitem__`` is automatically a class method
+if defined in Python code. To define this method in a C extension, one
+should use flags ``METH_O|METH_CLASS``. For example, a simple way to make
+an extension class generic is to use a method that simply returns the
+original class objects, thus fully erasing the type information at runtime,
+and deferring all check to static type checkers only::
+
+  typedef struct {
+      PyObject_HEAD
+      /* ... your code ... */
+  } SimpleGeneric;
+
+  static PyObject *
+  simple_class_getitem(PyObject *type, PyObject *item)
+  {
+      Py_INCREF(type);
+      return type;
+  }
+
+  static PyMethodDef simple_generic_methods[] = {
+      {"__class_getitem__", simple_class_getitem, METH_O|METH_CLASS, NULL},
+      /* ... other methods ... */
+  };
+
+  PyTypeObject SimpleGeneric_Type = {
+      PyVarObject_HEAD_INIT(NULL, 0)
+      "SimpleGeneric",
+      sizeof(SimpleGeneric),
+      0,
+      .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+      .tp_methods = simple_generic_methods,
+  };
+
+Such class can be used as a normal generic in Python type annotations
+(a corresponding stub file should be provided for static type checkers,
+see PEP 484 for details)::
+
+  from simple_extension import SimpleGeneric
+  from typing import TypeVar
+
+  T = TypeVar('T')
+
+  Alias = SimpleGeneric[str, T]
+  class SubClass(SimpleGeneric[T, int]):
+      ...
+
+  data: Alias[int]  # Works at runtime
+  more_data: SubClass[str]  # Also works at runtime
+
+
 Backwards compatibility and impact on users who don't use ``typing``
 ====================================================================