models: Change return type of get_human_admin_users to QuerySet.
models: Change return type of get_human_admin_users to QuerySet.

I would be accessing some methods of QuerySet in the subsequent
commits. So marking this as Sequence results in mypy errors.

hackerkid/zulip,zulip/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,rht/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,andersk/zulip,kou/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,rht/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,punchagan/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip