<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_parents():
 """Return sorted list of names of packages without dependants."""

    distributions = get_installed_distributions(user_only=ENABLE_USER_SITE)
    remaining = {d.project_name.lower() for d in distributions}
    requirements = {r.project_name.lower() for d in distributions for
                    r in d.requires()}

    return get_realnames(remaining - requirements)