how do i write a cross-platform relative file path?

[pseudo]

hi i'm current reading python crash course and it says that under unix-like platforms I have to use forward-slash like this:

with open('text_files/filename.txt') as file_object:

and under windows operating systems i have to use back-slash like this:

with open('text_files\filename.txt') as file_object:

but what if i wanted to create a multi-platform script? is there a universal way of writing file/folder paths?