Fix ref leak when calling update (#1135)

fixes #1134
appears to be introduced in #234 (4.4.0)

---------

Co-authored-by: Andrew Svetlov <[email protected]>

Author: bdraco

CHANGES/1135.bugfix.rst

@@ -0,0 +1,3 @@
+Fixed reference counting when calling :py:meth:`multidict.MultiDict.update` -- by :user:`bdraco`.
+
+The leak was introduced in 4.4.0

multidict/_multilib/pair_list.h

@@ -1015,6 +1015,7 @@ _dict_set_number(PyObject *dict, PyObject *key, Py_ssize_t num)
         return -1;
     }
 
+    Py_DECREF(tmp);
     return 0;
 }