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

(cherry picked from commit 1147810)

Co-authored-by: sobolevn <mail@sobolevn.me>

Author: sobolevn

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.