bpo-31497: Add private helper _PyType_Name(). (#3630)

This function returns the last component of tp_name after a dot.
Returns tp_name itself if it doesn't contain a dot.

Objects/exceptions.c

@@ -116,13 +116,7 @@ BaseException_str(PyBaseExceptionObject *self)
 static PyObject *
 BaseException_repr(PyBaseExceptionObject *self)
 {
-    const char *name;
-    const char *dot;
-
-    name = Py_TYPE(self)->tp_name;
-    dot = (const char *) strrchr(name, '.');
-    if (dot != NULL) name = dot+1;
-
+    const char *name = _PyType_Name(Py_TYPE(self));
     return PyUnicode_FromFormat("%s%R", name, self->args);
 }