bpo-35081: Add pycore_fileutils.h (GH-10371)

Move Py_BUILD_CORE code from Include/fileutils.h to a new
Include/internal/pycore_fileutils.h file.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 3692da6..5338781 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -40,6 +40,7 @@
 
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
+#include "pycore_fileutils.h"
 #include "pycore_state.h"
 #include "ucnhash.h"
 #include "bytes_methods.h"