Fix performance regression in exception tracebacks.

See #1719.

Cython/Utility/Exceptions.c

@@ -603,10 +603,19 @@ static void __Pyx_AddTraceback(const char *funcname, int c_line,
     PyObject *use_cline = 0;
     PyObject *ptype, *pvalue, *ptraceback;
 
+    static PyObject* cline_in_traceback = NULL;
+    if (cline_in_traceback == NULL) {
+      cline_in_traceback = PyString_InternFromString("cline_in_traceback");
+    }
+
     if (c_line) {
       PyErr_Fetch(&ptype, &pvalue, &ptraceback);
-      use_cline = PyObject_GetAttrString(${cython_runtime_cname}, "cline_in_traceback");
-      if (use_cline == NULL || PyObject_Not(use_cline) != 0) {
+      use_cline = PyObject_GetAttr(${cython_runtime_cname}, cline_in_traceback);
+      if (use_cline == NULL) {
+        c_line = 0;
+        PyObject_SetAttr(${cython_runtime_cname}, cline_in_traceback, Py_False);
+      }
+      else if (PyObject_Not(use_cline) != 0) {
         c_line = 0;
       }
       PyErr_Restore(ptype, pvalue, ptraceback);