bpo-1635741: _contextvars uses PyModule_AddType() (GH-23147)

Replace PyModule_AddObject() with PyModule_AddType() in the
_contextvars module (Python-ast.c).

Add also the module name to _contextvars types name.
diff --git a/Python/context.c b/Python/context.c
index 15d8b8e..82826bf 100644
--- a/Python/context.c
+++ b/Python/context.c
@@ -703,7 +703,7 @@
 
 PyTypeObject PyContext_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
-    "Context",
+    "_contextvars.Context",
     sizeof(PyContext),
     .tp_methods = PyContext_methods,
     .tp_as_mapping = &PyContext_as_mapping,
@@ -1056,7 +1056,7 @@
 
 PyTypeObject PyContextVar_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
-    "ContextVar",
+    "_contextvars.ContextVar",
     sizeof(PyContextVar),
     .tp_methods = PyContextVar_methods,
     .tp_members = PyContextVar_members,
@@ -1197,7 +1197,7 @@
 
 PyTypeObject PyContextToken_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
-    "Token",
+    "_contextvars.Token",
     sizeof(PyContextToken),
     .tp_methods = PyContextTokenType_methods,
     .tp_getset = PyContextTokenType_getsetlist,