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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 _get_credentials(self):
 # type: () -> Optional[Tuple[str, str]] """ Return HDX site username and password Returns: Optional[Tuple[str, str]]: HDX site username and password or None """

        site = self.data[self.hdx_site]
        username = site.get('username')
        if username:
            return b64decode(username).decode('utf-8'), b64decode(site['password']).decode('utf-8')
        else:
            return None