message_fetch: Replace SELECT coercion with .subquery().
message_fetch: Replace SELECT coercion with .subquery().

Fixes “SADeprecationWarning: Implicit coercion of SELECT and textual
SELECT constructs into FROM clauses is deprecated; please call
.subquery() on any Core select or ORM Query object in order to produce
a subquery object.”

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

kou/zulip,zulip/zulip,andersk/zulip,kou/zulip,kou/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,kou/zulip,zulip/zulip,andersk/zulip,kou/zulip,rht/zulip,rht/zulip,kou/zulip,rht/zulip