bpo-39487: Merge duplicated _Py_IDENTIFIER identifiers in C code (GH-18254)

Moving repetitive `_Py_IDENTIFIER` instances to a global location helps identify them more easily in regards to sub-interpreter support.
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index 3ec5a00..527693c 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -25,6 +25,8 @@
 extern "C" {
 #endif
 
+_Py_IDENTIFIER(open);
+
 /* External C interface */
 
 PyObject *
@@ -32,7 +34,6 @@
               const char *errors, const char *newline, int closefd)
 {
     PyObject *io, *stream;
-    _Py_IDENTIFIER(open);
 
     /* import _io in case we are being used to open io.py */
     io = PyImport_ImportModule("_io");
@@ -547,7 +548,6 @@
 PyFile_OpenCodeObject(PyObject *path)
 {
     PyObject *iomod, *f = NULL;
-    _Py_IDENTIFIER(open);
 
     if (!PyUnicode_Check(path)) {
         PyErr_Format(PyExc_TypeError, "'path' must be 'str', not '%.200s'",