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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:
 async def _auth_plain(self, username, password):
 """ Performs an authentication attempt using the PLAIN mechanism. Protocol: 1. Format the username and password in a suitable way ; 2. The formatted string is base64-encoded ; 3. The string 'AUTH PLAIN' and a space character are prepended to the base64-encoded username and password and sent to the server ; 4. If the server replies with a 235 return code, user is authenticated. Args: username (str):
 Identifier of the user trying to authenticate. password (str):
 Password for the user. Raises: ConnectionResetError: If the connection with the server is unexpectedely lost. SMTPAuthenticationError: If the authentication attempt fails. Returns: (int, str):
 A (code, message) 2-tuple containing the server response. """

        mechanism = "PLAIN"

        credentials = "\0{}\0{}".format(username, password)
        encoded_credentials = SMTP.b64enc(credentials)

        try:
            code, message = await self.do_cmd(
                "AUTH", mechanism, encoded_credentials, success=(235, 503)
            )
        except SMTPCommandFailedError as e:
            raise SMTPAuthenticationError(e.code, e.message, mechanism)

        return code, message