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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def encrypt_to_file(contents, filename):
 """ Encrypts ``contents`` and writes it to ``filename``. ``contents`` should be a bytes string. ``filename`` should end with ``.enc``. Returns the secret key used for the encryption. Decrypt the file with :func:`doctr.travis.decrypt_file`. """

    if not filename.endswith('.enc'):
        raise ValueError("%s does not end with .enc" % filename)

    key = Fernet.generate_key()
    fer = Fernet(key)

    encrypted_file = fer.encrypt(contents)

    with open(filename, 'wb') as f:
        f.write(encrypted_file)

    return key