Pin Cython < 3 until fixed upstream
    
    Cython 3 has broke compatibility with some code that previously built
    successfully, in this case:
    
    ```
    /builddir/build/BUILD/rdma-core-46.0/redhat-linux-build/pyverbs/device.c:14510:18: error: redefinition of '__Pyx_Enum_ibv_event_type_to_py'
    14510 | static PyObject *__Pyx_Enum_ibv_event_type_to_py(enum ibv_event_type __pyx_v_c_val) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /builddir/build/BUILD/rdma-core-46.0/redhat-linux-build/pyverbs/device.c:10701:18: note: previous definition of '__Pyx_Enum_ibv_event_type_to_py' with type 'PyObject *(enum ibv_event_type)' {aka 'struct _object *(enum ibv_event_type)'}
    10701 | static PyObject *__Pyx_Enum_ibv_event_type_to_py(enum ibv_event_type __pyx_v_c_val) {
          |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    ```