fix isolation level test

Author: vgvoleg

tests/test_connection.py

@@ -16,6 +16,7 @@ async def _test_isolation_level_read_only(
         isolation_level: str,
         read_only: bool,
     ) -> None:
+        connection.set_isolation_level("AUTOCOMMIT")
         async with connection.cursor() as cursor:
             with suppress(dbapi.DatabaseError):
                 await cursor.execute("DROP TABLE foo")
@@ -39,6 +40,8 @@ async def _test_isolation_level_read_only(
 
         await connection.rollback()
 
+        connection.set_isolation_level("AUTOCOMMIT")
+
         async with connection.cursor() as cursor:
             await cursor.execute("DROP TABLE foo")