bpo-37392: Remove sys.setcheckinterval() (GH-14355)

Remove sys.getcheckinterval() and sys.setcheckinterval() functions.
They were deprecated since Python 3.2. Use sys.getswitchinterval()
and sys.setswitchinterval() instead.

Remove also check_interval field of the PyInterpreterState structure.

Python/pystate.c

@@ -203,7 +203,6 @@ PyInterpreterState_New(void)
 
     memset(interp, 0, sizeof(*interp));
     interp->id_refcount = -1;
-    interp->check_interval = 100;
 
     PyStatus status = PyConfig_InitPythonConfig(&interp->config);
     if (_PyStatus_EXCEPTION(status)) {