Add API for static strings, primarily good for identifiers.
Thanks to Konrad Schöbel and Jasper Schulz for helping with the mass-editing.
diff --git a/Python/ceval.c b/Python/ceval.c
index 686a54d..04d3df1 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -372,6 +372,7 @@
 void
 PyEval_ReInitThreads(void)
 {
+    _Py_identifier(_after_fork);
     PyObject *threading, *result;
     PyThreadState *tstate = PyThreadState_GET();
 
@@ -392,7 +393,7 @@
         PyErr_Clear();
         return;
     }
-    result = PyObject_CallMethod(threading, "_after_fork", NULL);
+    result = _PyObject_CallMethodId(threading, &PyId__after_fork, NULL);
     if (result == NULL)
         PyErr_WriteUnraisable(threading);
     else