Apply suggestions from code review

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: serhiy-storchaka

Lib/test/test_capi/test_weakref.py

@@ -94,6 +94,7 @@ def test_pyweakref_newref(self):
         obj = Object()
         wr = newref(obj)
         self.assertIs(type(wr), weakref.ReferenceType)
+        # PyWeakref_NewRef() handles None callback as NULL callback
         wr = newref(obj, None)
         self.assertIs(type(wr), weakref.ReferenceType)
         log = []
@@ -112,6 +113,7 @@ def test_pyweakref_newproxy(self):
         obj = Object()
         wp = newproxy(obj)
         self.assertIs(type(wp), weakref.ProxyType)
+        # PyWeakref_NewProxy() handles None callback as NULL callback
         wp = newproxy(obj, None)
         self.assertIs(type(wp), weakref.ProxyType)
         log = []

Modules/_testlimitedcapi/weakref.c

@@ -1,7 +1,7 @@
 #include "pyconfig.h"   // Py_GIL_DISABLED
 #ifndef Py_GIL_DISABLED
-   // Need limited C API 3.13 for PyLong_AsInt()
-#  define Py_LIMITED_API 0x030d0000
+   // Need limited C API 3.5 for PyModule_AddFunctions()
+#  define Py_LIMITED_API 0x03050000
 #endif
 
 #include "parts.h"