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/descrobject.c b/Objects/descrobject.c
index b448ec6..cb7572b 100644
--- a/Objects/descrobject.c
+++ b/Objects/descrobject.c
@@ -1,6 +1,7 @@
 /* Descriptors -- a new, flexible way to describe attributes */
 
 #include "Python.h"
+#include "pycore_ceval.h"   // _Py_EnterRecursiveCall()
 #include "pycore_object.h"
 #include "pycore_pystate.h"
 #include "pycore_tupleobject.h"