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

Misc/NEWS.d/next/Core and Builtins/2019-06-16-02-38-25.bpo-37300.WJkgKV.rst

+Remove an unnecssary Py_XINCREF in classobject.c.

Objects/classobject.c

@@ -110,7 +110,7 @@ PyMethod_New(PyObject *func, PyObject *self)
     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);