bpo-31780: Fix incorrect error message for ',x', ',b', ',o' specs (#4002)

Patch by Pablo.
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 2df7494..397ae7f 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -32,11 +32,11 @@
 {
     if (presentation_type > 32 && presentation_type < 128)
         PyErr_Format(PyExc_ValueError,
-                     "Cannot specify ',' or '_' with '%c'.",
+                     "Cannot specify ',' with '%c'.",
                      (char)presentation_type);
     else
         PyErr_Format(PyExc_ValueError,
-                     "Cannot specify ',' or '_' with '\\x%x'.",
+                     "Cannot specify ',' with '\\x%x'.",
                      (unsigned int)presentation_type);
 }