bpo-41985: Add _PyLong_FileDescriptor_Converter and AC converter for "fildes". (GH-22620)

diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index 1c6ecaf..9b89448 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -223,6 +223,17 @@
     return fd;
 }
 
+int
+_PyLong_FileDescriptor_Converter(PyObject *o, void *ptr)
+{
+    int fd = PyObject_AsFileDescriptor(o);
+    if (fd == -1) {
+        return 0;
+    }
+    *(int *)ptr = fd;
+    return 1;
+}
+
 /*
 ** Py_UniversalNewlineFgets is an fgets variation that understands
 ** all of \r, \n and \r\n conventions.