Add pip requirements file for dependency management

Because the AES cipher is configured in CBC mode, it pads the plaintext to a multiple of block size. There are multiple padding schemes, but the one used here is PKCS5 padding: pad the plaintext with *n* bytes, each with the value *m* equal to the number of bytes added. In the example above, AES has padded the plaintext with 4 bytes (*n*), each with value 4 (*m*).