[SOT][3.13] split certain LOAD_ATTR in 3.13+ & replace get_items and flatten_items withflatten_inner_vars,while replacing special cases with inheritance & remove method_name attribute in MethodVariable

[SOT][3.13] split certain LOAD_ATTR in 3.13+ & replace get_items and flatten_items withflatten_inner_vars,while replacing special cases with inheritance & remove method_name attribute in MethodVariable #1284