bpo-36728: Remove PyEval_ReInitThreads() from C API (GH-13241)

Remove the PyEval_ReInitThreads() function from the Python C API.
It should not be called explicitly: use PyOS_AfterFork_Child()
instead.

Rename PyEval_ReInitThreads() to _PyEval_ReInitThreads() and add a
'runtime' parameter.
diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
index cdc73a3..7a3166e 100644
--- a/Include/internal/pycore_ceval.h
+++ b/Include/internal/pycore_ceval.h
@@ -24,6 +24,8 @@
     void *arg);
 PyAPI_FUNC(void) _PyEval_SignalAsyncExc(
     struct _ceval_runtime_state *ceval);
+PyAPI_FUNC(void) _PyEval_ReInitThreads(
+    _PyRuntimeState *runtime);
 
 #ifdef __cplusplus
 }