Fix issue #11432. if the stdin pipe is the same file descriptor as either stdout or stderr
in the _posixsubprocess C extension module it would unintentionally close the fds and raise
an error.

Modules/_posixsubprocess.c

diff --git a/Modules/_posixsubprocess.c b/Modules/_posixsubprocess.c
index 0f85da9..bf10cbb 100644
--- a/Modules/_posixsubprocess.c
+++ b/Modules/_posixsubprocess.c
@@ -99,10 +99,10 @@
     if (p2cread > 2) {
         POSIX_CALL(close(p2cread));
     }
-    if (c2pwrite > 2) {
+    if (c2pwrite > 2 && c2pwrite != p2cread) {
         POSIX_CALL(close(c2pwrite));
     }
-    if (errwrite != c2pwrite && errwrite > 2) {
+    if (errwrite != c2pwrite && errwrite != p2cread && errwrite > 2) {
         POSIX_CALL(close(errwrite));
     }