py/objstr: Use better msg in bad implicit str/bytes conversion exception

Instead of always reporting some object cannot be implicitly be converted
to a 'str', even when it is a 'bytes' object, adjust the logic so that
when trying to convert str to bytes it is shown like that.
This will still report bad implicit conversion from e.g. 'int to bytes'
as 'int to str' but it will not result in the confusing
'can't convert 'str' object to str implicitly' anymore for calls like
b'somestring'.count('a').

py/objstr.c

@@ -2065,9 +2065,10 @@ STATIC void bad_implicit_conversion(mp_obj_t self_in) {
     if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
         mp_raise_TypeError("can't convert to str implicitly");
     } else {
+        const qstr src_name = mp_obj_get_type(self_in)->name;
         nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
-            "can't convert '%s' object to str implicitly",
-            mp_obj_get_type_str(self_in)));
+            "can't convert '%q' object to %q implicitly",
+            src_name, src_name == MP_QSTR_str ? MP_QSTR_bytes : MP_QSTR_str));
     }
 }