pybind: make Cluster and ClusterVector bindings module-local

- After upgrading to pybind11 3, duplicate registration of cluster-related
  types across `_aare` and `_aare_cuda` started failing.
- Mark the `Cluster` and `ClusterVector` bindings as `py::module_local()` so
each extension owns its local registration.

Note: cluster objects from CPU and CUDA bindings are now distinct Python types.

python/src/bind_Cluster.hpp

@@ -23,7 +23,7 @@ void define_Cluster(py::module &m, const std::string &typestr) {
     auto class_name = fmt::format("Cluster{}", typestr);
 
     py::class_<Cluster<Type, ClusterSizeX, ClusterSizeY, CoordType>>(
-        m, class_name.c_str(), py::buffer_protocol())
+        m, class_name.c_str(), py::buffer_protocol(), py::module_local())
 
         .def(py::init([](CoordType x, CoordType y,
                          py::array_t<Type, py::array::forcecast> data) {

python/src/bind_ClusterVector.hpp

@@ -31,7 +31,8 @@ void define_ClusterVector(py::module &m, const std::string &typestr) {
     py::class_<ClusterVector<
         Cluster<Type, ClusterSizeX, ClusterSizeY, CoordType>, void>>(
         m, class_name.c_str(),
-        py::buffer_protocol())
+        py::buffer_protocol(),
+        py::module_local())
 
         .def(py::init()) // TODO change!!!