closes bpo-37300: Remove unnecessary Py_XINCREF in classobject.c. (GH-14120)

diff --git a/Objects/classobject.c b/Objects/classobject.c
index 2415ed1..f26a85c 100644
--- a/Objects/classobject.c
+++ b/Objects/classobject.c
@@ -110,7 +110,7 @@
     im->im_weakreflist = NULL;
     Py_INCREF(func);
     im->im_func = func;
-    Py_XINCREF(self);
+    Py_INCREF(self);
     im->im_self = self;
     im->vectorcall = method_vectorcall;
     _PyObject_GC_TRACK(im);