bpo-39947: Move Py_EnterRecursiveCall() to internal C API (GH-18972)

Move the static inline function flavor of Py_EnterRecursiveCall() and
Py_LeaveRecursiveCall() to the internal C API: they access
PyThreadState attributes. The limited C API provides regular
functions which hide implementation details.
diff --git a/Objects/methodobject.c b/Objects/methodobject.c
index 16abded..44ae00f 100644
--- a/Objects/methodobject.c
+++ b/Objects/methodobject.c
@@ -2,6 +2,7 @@
 /* Method object implementation */
 
 #include "Python.h"
+#include "pycore_ceval.h"   // _Py_EnterRecursiveCall()
 #include "pycore_object.h"
 #include "pycore_pyerrors.h"
 #include "pycore_pymem.h"