Issue #14437: Fix building the _io module under Cygwin.

Misc/NEWS

@@ -105,6 +105,8 @@ Extension Modules
 Build
 -----
 
+- Issue #14437: Fix building the _io module under Cygwin.
+
 - Issue #14387: Do not include accu.h from Python.h.
 
 - Issue #14359: Only use O_CLOEXEC in _posixmodule.c if it is defined.

Modules/_io/_iomodule.h

@@ -67,7 +67,7 @@ typedef struct {
     PyObject *filename; /* Not used, but part of the IOError object */
     Py_ssize_t written;
 } PyBlockingIOErrorObject;
-PyAPI_DATA(PyObject *) PyExc_BlockingIOError;
+extern PyObject *PyExc_BlockingIOError;
 
 /*
  * Offset type for positioning.