fix with new docker image

CMakeLists.txt

@@ -152,7 +152,7 @@ find_python_module(graph_tool   REQUIRED)
 find_python_module(pyopencl     REQUIRED)
 find_python_module(pyfftw       REQUIRED)
 find_python_module(gpyfft       REQUIRED)
-#find_python_module(backports.weakref REQUIRED) # python-backports.weakref
+find_python_module(backports.weakref REQUIRED) # python-backports.weakref
 #find_python_module(backports.functools-lru-cache REQUIRED) # python-backports.functools-lru-cache
 # --- MPI ---
 if(USE_MPI)