gh-149403: Fix `NameError` in `Lib/test/test_capi/test_getargs.py` (#149404)

Lib/test/test_capi/test_getargs.py

diff --git a/Lib/test/test_capi/test_getargs.py b/Lib/test/test_capi/test_getargs.py
index 0b2473b..bbc09e5 100644
--- a/Lib/test/test_capi/test_getargs.py
+++ b/Lib/test/test_capi/test_getargs.py
@@ -13,7 +13,7 @@
 try:
     import _testinternalcapi
 except ImportError:
-    _testinternalcapi = NULL
+    _testinternalcapi = None
 
 # > How about the following counterproposal. This also changes some of
 # > the other format codes to be a little more regular.