Simple secret key encryption with PyNaCl
It's not really a cryptography question, but I think the decryption would go better if you read the data from infile first (box works on strings of data, not filenames), so 

with open(infile, 'r') as in_file:
    encrypted = in_file.read()


and later:

decrypted = box.decrypt(encrypted)


where the nonce before it can go, as it is not used (the encrypted data contains the generated salt from encrypting).

Also the quotes on "outfile" can go. It's a variable.