bpo-39481: Implementation for PEP 585 (#18239)

This implements things like `list[int]`,
which returns an object of type `types.GenericAlias`.
This object mostly acts as a proxy for `list`,
but has attributes `__origin__` and `__args__`
that allow recovering the parts (with values `list` and `(int,)`.

There is also an approximate notion of type variables;
e.g. `list[T]` has a `__parameters__` attribute equal to `(T,)`.
Type variables are objects of type `typing.TypeVar`.

Objects/dictobject.c

@@ -3252,6 +3252,7 @@ static PyMethodDef mapp_methods[] = {
     {"copy",            (PyCFunction)dict_copy,         METH_NOARGS,
      copy__doc__},
     DICT___REVERSED___METHODDEF
+    {"__class_getitem__", (PyCFunction)Py_GenericAlias, METH_O|METH_CLASS, PyDoc_STR("See PEP 585")},
     {NULL,              NULL}   /* sentinel */
 };