Improve hack used in debug_options

src/sage/structure/debug_options.pyx

@@ -41,12 +41,14 @@ cdef class DebugOptions_class:
 
 cdef DebugOptions_class debug = DebugOptions_class()
 
-# Since "debug" is declared with a type, it can only be cimported at
+# Since "debug" is declared with cdef, it can only be cimported at
 # the Cython level, not imported in plain Python. So we add it to the
-# globals manually. We need to hack into Cython internals for this
-# since Sage is compiled with the old_style_globals option.
-from cpython.object cimport PyObject
-cdef extern from *:
-    PyObject* __pyx_d
-
-(<object>__pyx_d)["debug"] = debug
+# globals manually. However this will make the variable out of sync
+# if some user modifies the object, which is inevitable.
+# See https://github.com/cython/cython/issues/3959#issuecomment-753455240
+# and https://github.com/cython/cython/issues/656
+# Note that ``_this_module`` could not be ``globals()``
+# because Sage is compiled with the old_style_globals option.
+import sage.structure.debug_options as _this_module
+_this_module.debug = debug
+del _this_module