Fix libxml_PyFileGet with stdout on macOS

macOS returns O_RDWR for standard file descriptors, but fails to write
to stdout or stderr when opened with fdopen(dup_fd, "rw").

python/types.c

@@ -154,6 +154,19 @@ libxml_PyFileGet(PyObject *f) {
     if (!is_write && !is_read) /* also happens if we did not load or run NtQueryInformationFile() successfully */
         return(NULL);
 #else
+    /*
+     * macOS returns O_RDWR for standard streams, but fails to write to
+     * stdout or stderr when opened with fdopen(dup_fd, "rw").
+     */
+    switch (fd) {
+        case STDIN_FILENO:
+            mode = "r";
+            break;
+        case STDOUT_FILENO:
+        case STDERR_FILENO:
+            mode = "w";
+            break;
+        default:
             /*
              * Get the flags on the fd to understand how it was opened
              */
@@ -180,6 +193,7 @@ libxml_PyFileGet(PyObject *f) {
                 default:
                     return(NULL);
             }
+    }
 #endif
 
     /*