Since Python 3.10, Py_REFCNT() can no longer be used as an l-value. See

https://docs.python.org/3.10/whatsnew/3.10.html#id2

for details.

--- a/_ruamel_yaml.c
+++ b/_ruamel_yaml.c
@@ -23877,9 +23877,9 @@
   {
     PyObject *etype, *eval, *etb;
     PyErr_Fetch(&etype, &eval, &etb);
-    ++Py_REFCNT(o);
+    Py_SET_REFCNT(o, Py_REFCNT(o) + 1);
     __pyx_pw_12_ruamel_yaml_7CParser_3__dealloc__(o);
-    --Py_REFCNT(o);
+    Py_SET_REFCNT(o, Py_REFCNT(o) - 1);
     PyErr_Restore(etype, eval, etb);
   }
   Py_CLEAR(p->stream);
@@ -24050,9 +24050,9 @@
   {
     PyObject *etype, *eval, *etb;
     PyErr_Fetch(&etype, &eval, &etb);
-    ++Py_REFCNT(o);
+    Py_SET_REFCNT(o, Py_REFCNT(o) + 1);
     __pyx_pw_12_ruamel_yaml_8CEmitter_3__dealloc__(o);
-    --Py_REFCNT(o);
+    Py_SET_REFCNT(o, Py_REFCNT(o) - 1);
     PyErr_Restore(etype, eval, etb);
   }
   Py_CLEAR(p->stream);
--- a/_ruamel_yaml.h
+++ b/_ruamel_yaml.h
@@ -12,6 +12,10 @@
 #define PyString_GET_SIZE   PyBytes_GET_SIZE
 #define PyString_FromStringAndSize  PyBytes_FromStringAndSize
 
+#if PY_VERSION_HEX < 0x030900A4
+# define Py_SET_REFCNT(obj, refcnt) ((Py_REFCNT(obj) = (refcnt)), (void)0)
+#endif
+
 #endif
 
 #ifdef _MSC_VER	/* MS Visual C++ 6.0 */