Cleanup dangling reference in get_timezone_utc_capi (#5317)

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 3f41134..2ad4322 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -2300,8 +2300,10 @@
         return NULL;
     }
     if (macro) {
+        Py_INCREF(PyDateTime_TimeZone_UTC);
         return PyDateTime_TimeZone_UTC;
     } else {
+        Py_INCREF(PyDateTimeAPI->TimeZone_UTC);
         return PyDateTimeAPI->TimeZone_UTC;
     }
 }