bpo-40268: Remove unused osdefs.h includes (GH-19532)

When the include is needed, add required symbol in a comment.
diff --git a/Objects/exceptions.c b/Objects/exceptions.c
index 69a6b5c..ca917b4 100644
--- a/Objects/exceptions.c
+++ b/Objects/exceptions.c
@@ -9,7 +9,7 @@
 #include "pycore_initconfig.h"
 #include "pycore_object.h"
 #include "structmember.h"         // PyMemberDef
-#include "osdefs.h"
+#include "osdefs.h"               // SEP
 
 
 /* Compatibility aliases */
@@ -1435,11 +1435,13 @@
     size = PyUnicode_GET_LENGTH(name);
     offset = 0;
     for(i=0; i < size; i++) {
-        if (PyUnicode_READ(kind, data, i) == SEP)
+        if (PyUnicode_READ(kind, data, i) == SEP) {
             offset = i + 1;
+        }
     }
-    if (offset != 0)
+    if (offset != 0) {
         return PyUnicode_Substring(name, offset, size);
+    }
     else {
         Py_INCREF(name);
         return name;