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/unicodeobject.c b/Objects/unicodeobject.c
index 8470e41..11fa1fb5 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -40,6 +40,7 @@
 
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
+#include "pycore_bytes_methods.h"
 #include "pycore_fileutils.h"
 #include "pycore_initconfig.h"
 #include "pycore_object.h"
@@ -47,7 +48,6 @@
 #include "pycore_pylifecycle.h"
 #include "pycore_pystate.h"
 #include "ucnhash.h"
-#include "bytes_methods.h"
 #include "stringlib/eq.h"
 
 #ifdef MS_WINDOWS