bpo-11410: Standardize and use symbol visibility attributes across POSIX and Windows. (GH-16347)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index a5f4c47..b5d308a 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -1124,7 +1124,7 @@
     (releasebufferproc) bytesiobuf_releasebuffer,
 };
 
-PyTypeObject _PyBytesIOBuffer_Type = {
+Py_EXPORTED_SYMBOL PyTypeObject _PyBytesIOBuffer_Type = {
     PyVarObject_HEAD_INIT(NULL, 0)
     "_io._BytesIOBuffer",                      /*tp_name*/
     sizeof(bytesiobuf),                        /*tp_basicsize*/