Issue #15989: Fix several occurrences of integer overflow
when result of PyLong_AsLong() narrowed to int without checks.

This is a backport of changesets 13e2e44db99d and 525407d89277.
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index d20f196..500883e 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -200,7 +200,7 @@
     PyObject *meth;
 
     if (PyLong_Check(o)) {
-        fd = PyLong_AsLong(o);
+        fd = _PyLong_AsInt(o);
     }
     else if ((meth = PyObject_GetAttrString(o, "fileno")) != NULL)
     {
@@ -210,7 +210,7 @@
             return -1;
 
         if (PyLong_Check(fno)) {
-            fd = PyLong_AsLong(fno);
+            fd = _PyLong_AsInt(fno);
             Py_DECREF(fno);
         }
         else {