Please let us know which OS you are using? If it is ubuntu or any other flavor of Linux, cat command should just work fine. Let me tell you cat command has nothing to do with python version. It is an inbuilt Linux command. If you are using Windows, it is obvious Linux commands won't work on it. There is an alternative to cat command called type in windows, which works the same as cat command. The syntax for it is !type <filename>.
I hope this answers your question. Please let me know if you have any more doubts.