Make __Pyx_CoroutineAwaitType non-pickleable (GH-4381)

This is explicitly tested for: https://github.com/cython/cython/blob/aea4e6b84b38223c540266f8c57093ee2039f284/tests/run/test_coroutines_pep492.pyx#L2400

It turns out some earlier versions of Python assume that
C-API classes without a dict or slot are pickleable by the class
name. Currently it isn't pickleable because the class name lookup
is failing but this change makes it more robust.

See https://github.com/cython/cython/pull/4376

Cython/Utility/Coroutine.c

@@ -1501,6 +1501,22 @@ static PyObject *__Pyx_CoroutineAwait_no_new(PyTypeObject *type, PyObject *args,
 }
 #endif
 
+// In earlier versions of Python an object with no __dict__ and not __slots__ is assumed
+// to be pickleable by default. Coroutine-wrappers have significant state so shouldn't be.
+// Therefore provide a default implementation.
+// Something similar applies to heaptypes (i.e. with type_specs) with protocols 0 and 1
+// even in more recent versions.
+// We are applying this to all Python versions (hence the commented out version guard)
+// to make the behaviour explicit.
+// #if PY_VERSION_HEX < 0x03060000 || CYTHON_USE_TYPE_SPECS
+static PyObject *__Pyx_CoroutineAwait_reduce_ex(__pyx_CoroutineAwaitObject *self, PyObject *arg) {
+    CYTHON_UNUSED_VAR(arg);
+    PyErr_Format(PyExc_TypeError, "cannot pickle '%.200s' object",
+                         Py_TYPE(self)->tp_name);
+    return NULL;
+}
+// #endif
+
 static PyMethodDef __pyx_CoroutineAwait_methods[] = {
     {"send", (PyCFunction) __Pyx_CoroutineAwait_Send, METH_O,
      (char*) PyDoc_STR("send(arg) -> send 'arg' into coroutine,\nreturn next yielded value or raise StopIteration.")},
@@ -1508,6 +1524,11 @@ static PyMethodDef __pyx_CoroutineAwait_methods[] = {
      (char*) PyDoc_STR("throw(typ[,val[,tb]]) -> raise exception in coroutine,\nreturn next yielded value or raise StopIteration.")},
     {"close", (PyCFunction) __Pyx_CoroutineAwait_Close, METH_NOARGS,
      (char*) PyDoc_STR("close() -> raise GeneratorExit inside coroutine.")},
+// only needed with type-specs or version<3.6, but included in all versions for clarity
+// #if PY_VERSION_HEX < 0x03060000 || CYTHON_USE_TYPE_SPECS
+    {"__reduce_ex__", (PyCFunction) __Pyx_CoroutineAwait_reduce_ex, METH_O, 0},
+    {"__reduce__", (PyCFunction) __Pyx_CoroutineAwait_reduce_ex, METH_NOARGS, 0},
+// #endif
     {0, 0, 0, 0}
 };