responses = rally.get("Milestone", fetch=fields, project="Sample Project", order="FormattedID,TargetDate")
In order to scope to a specific project, it is instead necessary to use a query argument as follows:
responses = rally.get("Milestone", fetch=fields, project="Sample Project", order="FormattedID,TargetDate", query="((Projects contains \"/project/<PROJECT_OID>\") OR (Projects.ObjectID = null))")