17c4f47b0a updated table_kind to return
`pg_class.relkind`, however the semantics are *not* the same, and that
was ignored: `relkind = t` is for *toast* tables, not *temporary*
tables. In `pg_class` the temporary-ness is instead signaled by
`relpersistence` (which applies to both tables and sequences), temp
(and unlogged) tables have `relkind = r`. `existing_tables` does that
correctly, possibly unwittingly.
While at it, upgrade `table_kind` to return an `enum` (whose value is
the old discriminant).
closesodoo/odoo#117444
Related: odoo/enterprise#39185
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>