bpo-40876: Clarify error message in the csv module (GH-20653)

Misc/NEWS.d/next/Library/2020-06-05-20-00-18.bpo-40876.zDhiZj.rst

@@ -0,0 +1 @@
+Clarify error message in the :mod:`csv` module.

Modules/_csv.c

@@ -810,7 +810,7 @@ Reader_iternext(ReaderObj *self)
             PyErr_Format(_csvstate_global->error_obj,
                          "iterator should return strings, "
                          "not %.200s "
-                         "(did you open the file in text mode?)",
+                         "(the file should be opened in text mode)",
                          Py_TYPE(lineobj)->tp_name
                 );
             Py_DECREF(lineobj);