Move not null Oid to pgtype

In preparation to ConnInfo implementation.

values_test.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/jackc/pgx"
+	"github.com/jackc/pgx/pgtype"
 )
 
 func TestDateTranscode(t *testing.T) {
@@ -83,7 +84,7 @@ func TestJSONAndJSONBTranscode(t *testing.T) {
 	conn := mustConnect(t, *defaultConnConfig)
 	defer closeConn(t, conn)
 
-	for _, oid := range []pgx.Oid{pgx.JsonOid, pgx.JsonbOid} {
+	for _, oid := range []pgtype.Oid{pgx.JsonOid, pgx.JsonbOid} {
 		if _, ok := conn.PgTypes[oid]; !ok {
 			return // No JSON/JSONB type -- must be running against old PostgreSQL
 		}