bpo-35081: Move bytes_methods.h to the internal C API (GH-18492)

Move the bytes_methods.h header file to the internal C API as
pycore_bytes_methods.h: it only contains private symbols (prefixed by
"_Py"), except of the PyDoc_STRVAR_shared() macro.
diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index a3fc35c..d396435 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -2,11 +2,11 @@
 
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
+#include "pycore_bytes_methods.h"
 #include "pycore_object.h"
 #include "pycore_pymem.h"
 #include "pycore_pystate.h"
 #include "structmember.h"
-#include "bytes_methods.h"
 #include "bytesobject.h"
 #include "pystrhex.h"