Use `to_regtype($1)` instead of `$1::regtype` in TypeInfo.fetch query

[CaselIT]

Using $1::regtype will raise an error if the type does not exist. An example is if the hstore is not installed in the db.

ref sqlalchemy/sqlalchemy#8631 (comment) and sqlalchemy/sqlalchemy#8631 (comment)

cc @om26er

note: it seems to_regtype was added in 9.4 (at least the documentation of 9.3 does not mention it)