bpo-40593: Improve syntax errors for invalid characters in source code. (GH-20033)

diff --git a/Include/cpython/unicodeobject.h b/Include/cpython/unicodeobject.h
index 81a35cd..9432687 100644
--- a/Include/cpython/unicodeobject.h
+++ b/Include/cpython/unicodeobject.h
@@ -1222,6 +1222,8 @@
    and where the hash values are equal (i.e. a very probable match) */
 PyAPI_FUNC(int) _PyUnicode_EQ(PyObject *, PyObject *);
 
+PyAPI_FUNC(Py_ssize_t) _PyUnicode_ScanIdentifier(PyObject *);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/Include/errcode.h b/Include/errcode.h
index b37cd26..790518b 100644
--- a/Include/errcode.h
+++ b/Include/errcode.h
@@ -29,7 +29,6 @@
 #define E_EOFS          23      /* EOF in triple-quoted string */
 #define E_EOLS          24      /* EOL in single-quoted string */
 #define E_LINECONT      25      /* Unexpected characters after a line continuation */
-#define E_IDENTIFIER    26      /* Invalid characters in identifier */
 #define E_BADSINGLE     27      /* Ill-formed single statement input */
 
 #ifdef __cplusplus