Create a program that, using RSA public key cryptographic method, creates a pair of public and private keys, first encrypts a long string or a large number using the private key, writes the result to a file, then retrieves the cyphertext from the file and decrypts it using the public key. (If you are encrypting a number, it must be randomly generated).
(Note: in some cryptographic libraries, one can encrypt with public key only, and, respectively, decrypt with private key only; since both keys are interchangeable, you can always use public key as private and vice versa).
You can use any version of Python convenient for you. Use an appropriate library of tools for asymmetric encryption (if necessary, find such a library and install it) – learn how to use the tools in this library.