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/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index 36a27f4..a3719d8 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -115,7 +115,6 @@
     <ClInclude Include="..\Include\ast.h" />
     <ClInclude Include="..\Include\bitset.h" />
     <ClInclude Include="..\Include\boolobject.h" />
-    <ClInclude Include="..\Include\bytes_methods.h" />
     <ClInclude Include="..\Include\bytearrayobject.h" />
     <ClInclude Include="..\Include\bytesobject.h" />
     <ClInclude Include="..\Include\cellobject.h" />
@@ -161,6 +160,7 @@
     <ClInclude Include="..\Include\import.h" />
     <ClInclude Include="..\Include\internal\pycore_accu.h" />
     <ClInclude Include="..\Include\internal\pycore_atomic.h" />
+    <ClInclude Include="..\Include\internal\pycore_bytes_methods.h" />
     <ClInclude Include="..\Include\internal\pycore_call.h" />
     <ClInclude Include="..\Include\internal\pycore_ceval.h" />
     <ClInclude Include="..\Include\internal\pycore_code.h" />